Per Se-Inspired Golden Onion Tartlets

Thaw puff pastry according to package directions, typically 30-40 minutes at room temperature.

Heat butter and olive oil in a large skillet or Dutch oven over medium-low heat. Add the thinly sliced onions, a pinch of salt, and the optional sugar. Stir to coat.

Cook the onions slowly, stirring occasionally, for 40-50 minutes, or until deeply golden brown and caramelized. The key is low heat and patience to draw out their natural sweetness without burning. If they start to dry out, add a tablespoon of water. Stir in the chopped thyme and black pepper during the last 5 minutes of cooking. Taste and adjust seasoning.

Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

Unroll the thawed puff pastry on a lightly floured surface. Gently roll it out slightly, to about a 10x12-inch rectangle. Cut the pastry into 12-16 small squares or circles (about 2.5-3 inches each).

Carefully transfer the pastry pieces to the prepared baking sheet. Using a sharp knife, score a border about 1/4 inch from the edge of each pastry piece, being careful not to cut all the way through. This creates a crust for the filling.

Spoon a small amount of the caramelized onion mixture into the center of each pastry square, staying within the scored border.

Brush the exposed pastry edges with the whisked egg wash.

Bake for 18-25 minutes, or until the pastry is puffed, golden brown, and the onions are bubbling. Keep an eye on them to prevent over-browning.

Remove from the oven and let cool slightly on the baking sheet before serving warm or at room temperature.