Follow these simple steps to prepare your Caramelized Onion Tartlets:

1. Preheat the oven to 400°F (200°C).
2. Caramelize the onions: In a pan, heat olive oil over medium heat. Add the thinly sliced onions, sugar, salt, and pepper. Stir occasionally and cook until the onions are soft and golden, which should take about 15-20 minutes.
3. Prepare the puff pastry: Roll out the puff pastry on a lightly floured surface. Cut the pastry into small squares and place them on a baking sheet lined with parchment paper.
4. Assemble the tartlets: Spread a generous amount of goat cheese onto each pastry square. Top each with a spoonful of the caramelized onions.
5. Bake the tartlets: Place the baking sheet in the preheated oven and bake for 15-20 minutes or until the pastry is golden and puffed.
6. Garnish and serve: Remove the tartlets from the oven. Garnish with fresh thyme if desired, then serve warm.