In a large skillet, combine olive oil and butter over medium heat. Once the butter melts, add onions, salt, and pepper. Cook undisturbed for about 5 minutes.

Sprinkle in sugar and continue to cook for another 15 minutes until the onions turn a deep golden brown.

Add balsamic vinegar and thyme, stir to combine, and cook for another 5 minutes until the vinegar thickens slightly.

Preheat your oven to 400°F (200°C). Roll out puff pastry to about ¼ inch thickness and cut into rectangles or squares.

Spread a layer of caramelized onions over each pastry piece and crumble goat cheese on top.

Bake the tarts for 15 minutes until golden brown, checking every 2 minutes after the first 10 minutes.

Allow the tarts to cool on the baking sheet for about 5 minutes before serving.