This onion tart is made up of two versatile parts, both of which can be adapted for many other uses. The flaky pastry crust can be used for sweet or savory pies or a galette. The caramelized onions are incredibly rich and decadent. The onions can also be used in a brothy soup, with an omelet, on toast, or as a topping for a burger.

Flaky Pastry

This recipe makes enough dough for 1 single crust 9 inch tart, pie, or galette. Double recipe if making a pie that requires a top and bottom crust.

1 ¼ cups (160 g) all-purpose flour, plus more for dusting
1 tsp (6 g) kosher salt
5 oz unsalted butter, diced and refrigerated
¼ -½ cup ice water

Egg wash:
1 egg
A dash of cream

Combine the flour and salt in a bowl. Add the butter to the bowl and use your fingers to break the butter pieces into smaller pieces, about the size of a pea. It is okay to have a few larger pieces as long as the butter stays very cold. Slowly add the water, mixing with your hands until the dough begins to form. The dough should be very shaggy and dry, if all of the dough will not come together add a little more water.

Working quickly so the butter does not begin to melt, turn the dough out onto a lightly floured surface and shape it into a ball. Wrap the dough in plastic wrap and rest in the refrigerator for 30 minutes. Remove dough from the refrigerator and flatten into a flat disc. Wrap the dough back in the plastic wrap and place in the refrigerator for 1 hour. The dough can keep in the refrigerator for up to 3 days or it can be frozen for 3 months. If frozen, thaw the dough in the refrigerator the night before using.

Caramelized Onion Filling

6 large yellow onions, very thinly sliced
6 oz unsalted butter, cut into 1 inch pieces
Kosher salt
Sugar
2 sprigs of thyme
2 sprigs of tarragon
½ cup dry white wine

2 egg yolks (if using for tart)
¼ cup heavy cream (if using for tart)

Preheat oven to 450 degrees.

In a large pot or ductch oven combine the sliced onions, butter, salt, sugar, thyme and tarragon. Place the uncovered pot in the preheated oven. Roast onions for five minutes, then carefully stir the pot being sure to scrape down the sides and bottom of the pot. Continue to cook the ovens for 20 minutes, stirring every 5 or so minutes. When onions appear translucent and soft add half of the white wine, stir and continue to cook for another 10 minutes, checking on and stirring the onions as needed. Once most of the wine has evaporated add the remaining wine and continue to cook and stir. The onions should become very soft and a deep brown but not crispy or burnt. Once the onions are very soft, almost melting and reduced, remove them from the oven and allow to cool slightly. If using onions for anything other than the tart stop at this step and store onions for use later.

Lightly dust a work surface and rolling pin with flour. Roll out the crust into a 10 inch round. Using the rolling pin transfer the crust to a 9 inch shallow tart pan with a removable bottom. Drape the crust over the pan and gently press the crust against the sides of the pan and up to the top, being certain to push the crust in where the top and bottom of the pan meet. Trim off any excess dough and use scraps to patch any holes. Prick crust all over with a fork. Place the tart pan in the freezer for at least 30 minutes. Remove pan from freezer, line tart shell with a piece of foil or parchment paper, then fill shell with pie weights, beans or rice. Bake tart shell for 20 minutes, until sides are a light brown.

While the onions are cooling, heat the cream over low heat until warm. Slowly drizzle warm cream over the egg yolks while whisking. Be careful not to add too much of the cream at once or the yolks will begin to cook and scramble. Continue whisking until the eggs and cream are fully combined and the mixture begins to thicken slightly.

Transfer onions from the pot to a medium size bowl, remove any large herb sprigs. Fold warm cream mixture into the onions.

Spoon onions into the baked tart shell, spreading onions evenly throughout. Brush edges of the crust with egg wash.

Bake tart for 20 minutes or until the crust is golden brown and onions are beginning to crisp on the surface.

Allow the tart to cool for five minutes before removing from the pan. Cut tart into slices. Serve tart plain, with a poached egg, topped with creme fraiche, or just with a squeeze of lemon. The tart can be served warm, cold or can be reheated in the oven at a later time.

NOTE
If you do not have a tart shell you could easily use make a galette instead. To make the galette roll out the dough as you would if you were using a tart pan. Transfer the dough to a baking sheet lined with parchment paper. Scoop the onion filling into the center of the dough, spread into a smooth layer, leaving one inch of dough all the way around. Fold the uncovered edges over the filling, leaving a large portion of the filling uncovered. Brush the crust with egg wash, bake as you would for a tart.