01 -
Heat olive oil and butter in a large skillet over medium heat. Add sliced onions, sugar, salt, and pepper. Cook, stirring occasionally, allowing 5 minutes between stirs to develop color. After 15 minutes, stir in balsamic vinegar and thyme, then continue cooking for an additional 5 minutes until onions are jammy and deeply caramelized.
02 -
Roll out the puff pastry sheet on a lightly floured surface and cut into six equal rectangles. Place on a baking sheet lined with parchment paper.
03 -
Evenly distribute the caramelized onions over each pastry rectangle, leaving a 1/2-inch border around the edges. Generously crumble goat cheese atop the onions and sprinkle with additional fresh thyme leaves.
04 -
Preheat the oven to 400°F (204°C). Bake the tarts on the middle rack for 15 to 20 minutes, or until the pastry is puffed and golden brown.