French Onion Tartlets

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

Melt 2 tablespoons of unsalted butter in a large skillet over medium-low heat. Add the thinly sliced yellow onions and cook slowly for 15-20 minutes, stirring occasionally, until they are deeply caramelized and soft. Season with 1/4 teaspoon salt and 1/8 teaspoon black pepper.

Unroll the thawed puff pastry sheet onto a lightly floured surface. Cut the pastry into four equal squares.

Place the pastry squares on the prepared baking sheet. Score a border about 1/2 inch from the edge of each square with a knife, being careful not to cut all the way through.

Divide the caramelized onions evenly among the centers of the pastry squares, spreading them within the scored borders. Sprinkle 1/2 cup of shredded Gruyère cheese over the onions.

Brush the pastry borders with the beaten egg wash.

Bake for 20-25 minutes, or until the pastry is golden brown and puffed, and the cheese is bubbly. Let cool slightly before serving.