[Photographs: Vicky Wasik]

The arrival of colder weather means it's time to get back to comforting hearty food, and the French have that category covered. I've started planning my meals for an upcoming trip to Paris over the holidays, and my mind started wandering to classic bistro dishes like steak au poivre.

These daydreaming moments often end up as recipe brainstorming sessions for me, and I soon had an idea stuck in my mind: what if I mashed up French onion soup and classic tarte Tatin into a savory onion tart with the flavors of the classic soup?

Full disclosure: I don't love French onion soup or apple tarte Tatin. I usually find the soup to be way too heavy, and I'm over it after a few bites. And I'm not a big fan of soft, cooked apples, especially when they get sweetened even more with caramel. But this tart idea I could get behind.

I love the interplay of bitter and sweet in caramelized onions, and who doesn't love a buttery tart crust? Throw in some gooey Gruyère and meaty stock and I am all in. So I got to work to figure out the best way to make this delicious Francostein monster.

The Crust: Pie or Puff?

For the crust of a tarte Tatin, you can decide whether to make dough from scratch, or take the quick and easy route by using frozen puff pastry. I'm not a professional baker, nor am I here to throw shade at the store-bought crowd, but I will happily push the BraveTart agenda: Stella's old-fashioned flaky pie dough makes the best French onion tarte Tatin.

I tested with both throughout the recipe development process, and tasters unanimously preferred tarts made with pie dough over puff pastry. The pie dough tarts were more cohesive; the crust has structure and a pleasant chew, and it melds seamlessly with the caramelized onions and melty Gruyère cheese.

The puff pastry versions are a little more disjointed: The onions don't set in the pastry as well during the baking process, and the less-porous puff pastry traps more steam from the onions, which hinders browning and makes them softer in the process. That said, the tart is still real tasty even when made with puff pastry. And the process for preparing the crust is pretty much the same for both pie dough and puff pastry.

I start by rolling out the dough (half a batch of Stella's pie dough or one sheet of thawed puff pastry) on a well-floured board until it's large enough to cut out a 10-inch round. An inverted skillet (the same one you will use for the tart itself) is a good guide for trimming the dough circle.

Brush off excess flour from the dough and, with the help of your rolling pin, transfer the round to an inverted baking sheet lined with parchment paper.

Working around the edge of the dough, fold over a 1/2-inch border of dough, pinching it together to form a raised rim, which will help keep the onions contained and in place for the tart. Once you have completed this step, your crust should be 9 inches in diameter.

It's important to "dock" the dough (baking-speak for poking holes in it) to allow some steam to escape during baking; cut three slits in the crust if using pie dough, and for puff pastry you can poke the round all over with a fork. Pop the sheet tray in the fridge to chill the dough while you work on the onion filling.

The Onions: Sweet, Small, or Savory?

For his French onion soup recipe, Daniel recommends going with a mix of allium varieties for a soup with the most complex flavor. I briefly entertained the idea of trying to mix and match alliums for this tart, but realized early on that it would just further complicate an already involved recipe. I needed to simplify as much as possible and go with one type of onion, so I began testing with Daniel's recommended all-purpose variety: yellow onions.

Unlike Goldilocks, I quickly realized that my first choice was just right. Yellow onions have a middle-of-the-road allium flavor, neither too sweet nor too harsh, and a relatively low moisture content that makes them great for caramelizing, and their manageable size ended up being ideal for tart-building.

After spending way too long peeling dozens of cute cipolline and shallots for initial test tarts, I knew it would be cruel to subject people to that tear-inducing monotony. I wanted something more appealing and less a-peeling. With tiny alliums out of contention, I ran a side-by-side test comparing tarts made with yellow onions and Vidalias, and as suspected, the Vidalia tarte Tatin veered into too-sweet territory.

The Filling: Caramel or Caramelized?

Once I had the winning onion, I had to figure out the amount to use, and three small-ish yellow onions ended up fitting the bill (look for onions that weight about half a pound each, and are about 3 inches in diameter). I knew that I needed larger onion pieces with a flat surface to stand in for the fruit in a classic Tatin, so I started by cutting two onions into wedges through the root end, keeping the root attached but trimming off the scraggly part at the very end. Two onions will give you 16 wedges, the perfect amount for fitting into a 10-inch skillet.

In a classic Tatin, the fruit releases pectin-rich juices as it cooks on the stovetop, creating a glossy, sticky caramel with the butter and sugar in the skillet, which then binds the fruit together in the tart. Onions aren't as rich in pectin as apples, and don't cook down as readily, so I needed to find a way to recreate that binding effect without making the tart overly sweet in the process.

For my first round of testing, I tried two different methods to achieve this result. One was to make a wet caramel, using onion juice (purée raw onions with a little water, or use a juicer) instead of water, that I then loosened with chicken stock. This was a little trick I had picked up from one of the restaurants I worked at, where we made an onion caramel-chicken jus.

When the onion caramel is combined with an intense restaurant-style jus, everything works together—the sweetness of the caramel balanced by the savory intensity of the ultra-reduced, wine-fortified stock. This quick at-home version ended up being too sweet, because store-bought chicken stock doesn't have enough savory intensity to stand up to the caramel. This tart wasn't meant to be dessert, so I ditched the onion caramel.

My second approach was to make a jammy caramelized onion filling to nestle into the negative space between the onion wedges in the tart. I needed to also find a way to incorporate the other flavors of French onion soup—meaty stock, sherry, thyme—into the tart, and this caramelized onion mixture proved to be the vehicle for those flavors, too.

Start by thinly slicing the third onion, and sweating it in a saucepan with a little butter. Normally, I like to take the time to slowly cook a proper batch of caramelized onions, but when you're only cooking one onion, you can speed up the process.

Lightly season the onion with salt to help coax out its liquid, and cook it over medium-high heat, stirring frequently. The onions will begin to stick and form a brown fond on the bottom of the pan, which you'll loosen by adding a tablespoon or two of water at a time and scraping up the brown bits from the bottom and sides of the pot.

Keep repeating this process until the onions are soft, sweet, and a deep golden brown, and then deglaze them one last time with dry sherry instead of water. Next, add in some stock (chicken or beef, more on that in a minute), and cook it down until the onions are jammy, and the liquid has reduced to just coat the onions.

Finish them with a splash of cider vinegar and fish sauce (an optional background savory boost), and some chopped thyme leaves. Once there is very little liquid left in the saucepan, set the onions aside to cool while you build the tart.

The Stock: Chicken or Beef?

A quick aside on what kind of stock you should use for this tart. The short answer: whichever meaty stock you have available. I tested with our homemade beef stock as well as with two varieties of store-bought chicken and beef stocks.

They all worked, and tasters were not able to distinguish between them, which makes sense seeing as there is very little stock that goes into the onions (compared with the amount that goes into a batch of French onion soup). For store-bought stock, just make sure you go with a low-sodium option, or else you will have to be much more careful with seasoning so that the tart doesn't end up too salty.

The Skillet: Stainless Steel or Cast Iron?

For building the tart, you will need a 10-inch skillet that needs to be oven-safe. You can decide whether to use cast iron or stainless steel, depending on your eyesight and arm-strength. You need to be able to keep an eye on how dark the sugar gets as it caramelizes in the skillet, and this is much easier to track on the shiny grey of stainless steel than on the black matte finish of cast iron.

After baking, you also need to be able to invert the skillet over a plate. Cast iron is a lot heavier than stainless steel, so it's worth testing your comfort with some skillet bicep curls before you start building your tart.

Despite being heavier and harder to see into, cast iron does achieve deeper and more even caramelization on the onions in the tart. If you can bear the weight, I'd recommend going with cast iron. Stainless steel won't let you down though, and can still give you excellent tart results. Unfortunately, due to their flared shape, 10-inch carbon steel skillet don't have as much surface area as most stainless or cast iron skillets, so they can't be swapped in without altering the recipe.

Once you have your skillet of choice, smear it with a few tablespoons of softened butter and then evenly sprinkle one tablespoon of sugar over the butter.

After a light seasoning of salt and pepper, I arrange the onion wedges in a tight formation in the skillet, making sure they are in even contact with the pan. Arranging them in an unheated pan is way easier than trying to nestle in the onions after melting the butter and sugar together in a skillet. As you may have noticed in one of the photo collages above, I tested tarts using Stella's toasted sugar to see if it would lead to deeper flavor and more even caramelization. With only one tablespoon of sugar in the recipe, the effects weren't noticeable. If you have a batch of toasted sugar kicking around in your pantry, you can certainly use it, but it won't make or break the tart.

Cook the onion wedges on high heat, without stirring the skillet, until the sugar turns a deep amber color. Stove burners don't heat evenly, so you will need to move the skillet around to promote even browning.

Spoon the caramelized onion mixture in the spaces between the onion wedges, and continue cooking just long enough to meld everything together.

Top the onions with a handful of shredded Gruyère, and then pop the dough lid over the whole deal. You don't want the crust to be sticking to the sides of the skillet, as that will make it hard to unmold after baking.

If you want some subtle acidity and bite in your tart, you can spread some Dijon mustard on the underside of the crust before you place it in the skillet. I tested the tarts with and without mustard, and liked how the subtle heat of the mustard offsets some of the sweetness from the onions.

The Bake and the Flip: Golden or Dark, Cool or Warm?

I found that it's important to bake the crust to a deep golden brown for this tart. If you pull it out of the oven while the crust is still blond, it will turn soggy once the tart has been inverted and has to support the weight of the onions. So let the crust take on plenty of color before you take the tart out of the oven.

Unfortunately, the most stressful part of the recipe comes at the very end, when it's time to invert the skillet to release the tart onto a plate. While it may be tempting to let the tart cool completely in the skillet before attempting the flip, don't. If you let the tart cool for too long, the sugar will harden, and unmolding will become a lot trickier.

Let the skillet cool for just a few minutes, and then get your flip on. The key here is confidence. Know that it will release, and you will be a tart champion. Get a large plate, secure it over the skillet, and then invert it confidently. Let gravity do it's thing, rather than trying to shake it loose if it doesn't release instantly. You'll feel it when the plate gets heavier, and you uncover your masterpiece.

As with your bags in an overhead luggage bin, some onions may have shifted during flight, so now is the time to rearrange any that have fallen out of place. Slide the tart onto a wire rack to finish cooling (otherwise you risk steaming the bottom crust). Once the tart has cooled down, you're good to slice, but make sure you show off your tart skills to guests before serving. Then blow everyone away with your French soup in a tart costume.

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