Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large skillet, heat the olive oil over medium heat. Add the thinly sliced onions and sprinkle with a pinch of salt. Sauté for about 15-20 minutes until the onions are soft, golden, and caramelized.
Once the onions are caramelized, pour in the balsamic vinegar and add the thyme. Stir to combine and cook for an additional 2-3 minutes until the vinegar reduces slightly. Season with salt and pepper to taste, then remove from heat and let cool slightly.
On a lightly floured surface, roll out the puff pastry to make it uniformly thin. Cut the pastry into squares or circles, roughly 4-5 inches in diameter.
Place the pastry pieces onto the prepared baking sheet and use a fork to gently prick the center of each pastry.
In a small bowl, mix the crumbled goat cheese with the heavy cream until smooth and creamy. Spoon a dollop of this mixture onto the center of each pastry, leaving a small outer border.
Spoon a hearty portion of the caramelized onion mixture on top of each dollop of goat cheese.
Brush the outer edges of the pastry with the beaten egg for a golden color once baked.
Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the pastry has puffed and is golden brown.
Once baked, remove the tarts from the oven and let them cool slightly before serving.
Notes
Serve on a wooden board or colorful platter, garnished with fresh thyme and a drizzle of balsamic glaze.