Step 1: Prepare the Caramelized Onions : Heat butter and olive oil in a skillet over medium heat. Add sliced onions and stir to coat with the butter-oil mixture. Sprinkle with salt and sugar (if using). Cook slowly over low-medium heat for about 25–30 minutes, stirring occasionally, until onions become golden brown and caramelized. Stir in thyme, black pepper, and balsamic vinegar. Remove from heat and let cool slightly.

Step 2: Prepare the Tart Shells : Preheat the oven to 375°F (190°C). Roll out the pie crust on a lightly floured surface. Use a 3-inch round cutter to cut circles from the dough. Press each circle into a greased mini muffin tin or tartlet pan. Prick the base of each tartlet with a fork to prevent puffing.

Step 3: Pre-Bake the Tart Shells (Optional) : Bake the shells for about 8 minutes until lightly golden. Remove from oven and let cool slightly.

Step 4: Assemble the Tartlets : Spoon a small amount of caramelized onions into each tart shell. Top with a sprinkle of goat cheese or Gruyère. Return the tartlets to the oven.

Step 5: Bake the Tartlets : Bake for 10–12 minutes, until the cheese is melted and the crust is golden brown. Cool slightly before serving.