Step by step

Method

4 steps

1. 1

   Heat 2 tablespoons olive oil in a wide skillet over medium-low. Add 3 large thinly sliced yellow onions and ½ teaspoon kosher salt. Cook, stirring often, 25–30 minutes until deeply soft and golden — they should look jammy, not fried crisp.

2. 2

   Stir in ¼ teaspoon black pepper, 1 teaspoon fresh thyme leaves, and 1 tablespoon balsamic vinegar. Cool 5 minutes so the onions don't wilt the pastry.

3. 3

   Heat the oven to 400°F. Unfold 1 sheet thawed frozen puff pastry on a parchment-lined sheet pan. Score a ½-inch border with a knife without cutting all the way through.

4. 4

   Spread the onions inside the border. Brush the pastry border with 1 beaten egg. Bake 20–25 minutes until the pastry is puffed and deep golden. Cool a few minutes, slice, and serve warm. You want a golden onion tart with sweet jammy onions — not soggy pale pastry or raw sharp onion.