French Onion Tart

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.