1 sheet store-bought frozen puff pastry, defrosted
1/4 cup butter, melted
1 large red onion, cut into rings (leave the rings intact)
1 large sweet onion, cut into rings (leave the rings intact)
2 tablespoons brown sugar
8 ounces goat cheese, crumbled
10 fresh thyme sprigs
Salt and pepper, to taste
Preheat the oven to 400 F.
Line a baking sheet with parchment paper. On a floured surface, roll out puff pastry to approximately a 9" x 13" rectangle, then transfer puff pastry to prepared baking sheet and prick with a fork.
Brush pastry with melted butter. Sprinkle with goat cheese and scatter thyme sprigs on top. Place onions over goat cheese, leaving a 1/2" border around the edges.
Bake until puff pastry is golden brown and crisp and cheese is warm and melted, about 30 minutes. Let cool slightly, then cut into squares and serve.