Yellow onions — 4 large (about 3 lb), thinly sliced
Unsalted butter — 3 Tbsp
Olive oil — 1 Tbsp
Fresh thyme leaves — 2 tsp (plus extra to finish)
Balsamic vinegar — 2 tsp
Kosher salt and freshly ground black pepper
Fresh figs — 12, thinly sliced
Egg — 1, beaten (for egg wash)
Flaky sea salt to finish
Instructions
Caramelize onions: In a large skillet over medium‑low heat, melt butter with olive oil. Add sliced onions and a generous pinch of salt; cook, stirring often, until deep golden and jammy, 40–45 minutes. Stir in thyme and balsamic; cool to room temp.
Heat oven to 400°F. Line 2 sheet pans with parchment. On a lightly floured surface, unfold puff pastry and cut into ~2‑inch squares (or rounds). Transfer to pans and dock centers with a fork.
Brush pastry edges with beaten egg. Spoon about 1 teaspoon caramelized onions into the center of each piece and top with a thin slice of fig. Season with pepper.
Bake until puffed and deeply golden, 15–18 minutes, rotating pans once. Sprinkle with a few fresh thyme leaves.
Serve warm or at room temperature. Makes ~32 tartlets for 8 servings.