French Onion Tart Recipe | Bon Appétit

Step 1

Place a rack in decrease third of oven; preheat to 400°. Mix 1½ lb. onions (about 4 small), ideally yellow, halved, thinly sliced, 3 Tbsp. unsalted butter, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a big Dutch oven or different heavy pot over medium, cowl, and cook dinner, stirring sometimes, till onions are softened and translucent, 9–11 minutes. Uncover and proceed to cook dinner, including splashes of water (as much as ½ cup) and stirring and scraping backside of pot usually, till deep golden brown, 15–20 minutes.

Step 2

Take away pot from warmth and stir 1 garlic clove, finely chopped, 2 tsp. sherry vinegar or pink wine vinegar, 2 tsp. Worcestershire sauce, 1 tsp. freshly floor pepper, and 2 oz. Gruyère, coarsely grated, into onion combination. Style and season with extra salt if wanted; put aside.

Step 3

In the meantime, if utilizing a 14-oz. bundle of frozen puff pastry, minimize dough in half and roll 1 half right into a 10×8″ rectangle on a chunk of parchment paper; reserve remaining half for one more use. If utilizing a 17.3-oz. bundle of frozen puff pastry, roll out 1 sheet on a chunk of parchment paper simply to easy out creases.

Step 4

Frivolously rating a ½” border round edges of rolled out
pastry (you don’t need to minimize very deeply; simply depart a light-weight mark), then prick in a number of
spots with a fork, staying inside border. Slide pastry with parchment paper onto a big
baking sheet. Bake till puffed and golden, 15–18 minutes. Take away crust from oven and
instantly press down on area inside border with a kitchen towel to flatten barely.

Step 5

Utilizing an offset spatula, unfold 1 Tbsp. Dijon mustard evenly over pastry, staying inside border. Spoon reserved onion combination over mustard and easy out evenly over floor. Scatter remaining 2 oz. Gruyère, coarsely grated, on prime.

Step 6

Bake tart till pastry is golden brown and cheese is melted, 10–quarter-hour. Let cool 5 minutes, then minimize tart into squares and prime with chopped chives.