With a generous layer of caramelized onions and a tantalizing cheese pull, this tart packs the concentrated depth of French onion soup into every bite. Everyone has a hack for caramelizing onions. Baking soda. A pinch of sugar. A little balsamic. The fact remains: Onions only properly caramelize when most of the water in them has evaporated. Pre-salting the onions and starting them in a covered pot helps draw out their liquid much quicker than any other method. The resulting steam tames the onions into a soft, sweet tangle before a final blast of heat quickly cooks off the liquid. Adding water, ironically, is the secret to even caramelization. Pouring in judicious splashes toward the end of cooking dislodges browning sugars from the bottom of the pot, ensuring beautifully burnished, never burnt onions. Flaky puff pastry is the ideal canvas for showing off your work. A swipe of mustard and a shower of nutty Gruyère amplify those flavors you methodically built in the pan. Cut the tart into small squares for a two-bite appetizer, or serve larger slices alongside a green salad for dinner.
4 servings
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3
¾
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2
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1
Place a rack in lower third of oven; preheat to 400°. Combine 1½ lb. onions (about 4 small), preferably yellow, halved, thinly sliced, 3 Tbsp. unsalted butter, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a large Dutch oven or other heavy pot over medium, cover, and cook, stirring occasionally, until onions are softened and translucent, 9–11 minutes. Uncover and continue to cook, adding splashes of water (up to ½ cup) and stirring and scraping bottom of pot often, until deep golden brown, 15–20 minutes.
Remove pot from heat and stir 1 garlic clove, finely chopped, 2 tsp. sherry vinegar or red wine vinegar, 2 tsp. Worcestershire sauce, 1 tsp. freshly ground pepper, and 2 oz. Gruyère, coarsely grated, into onion mixture. Taste and season with more salt if needed; set aside.
Meanwhile, if using a 14-oz. package of frozen puff pastry, cut dough in half and roll 1 half into a 10×8″ rectangle on a piece of parchment paper; reserve remaining half for another use. If using a 17.3-oz. package of frozen puff pastry, roll out 1 sheet on a piece of parchment paper just to smooth out creases.
Lightly score a ½” border around edges of rolled out pastry (you don’t want to cut very deeply; just leave a light mark), then prick in a few spots with a fork, staying within border. Slide pastry with parchment paper onto a large baking sheet. Bake until puffed and golden, 15–18 minutes. Remove crust from oven and immediately press down on space within border with a kitchen towel to flatten slightly.
Using an offset spatula, spread 1 Tbsp. Dijon mustard evenly over pastry, staying within border. Spoon reserved onion mixture over mustard and smooth out evenly over surface. Scatter remaining 2 oz. Gruyère, coarsely grated, on top.
Bake tart until pastry is golden brown and cheese is melted, 10–15 minutes. Let cool 5 minutes, then cut tart into squares and top with chopped chives.
