Heat olive oil in a skillet over medium heat. Add sliced onions and cook until caramelized, about 20 minutes.

Preheat the oven to 400°F (200°C). Roll out the puff pastry and cut into squares. Place on a baking sheet.

Spread the caramelized onions over the pastry, then sprinkle with crumbled goat cheese.

Drizzle with balsamic vinegar and season with salt and pepper.

Bake in the preheated oven for 15-20 minutes, or until the pastry is golden brown.