1- Preheat the oven to 200 C (400 F).
2- Place the tartlets on a baking sheet and let them thaw. Meanwhile, in a pan, heat the olive oil. Add the onions and cook for a few minutes over medium heat.
3- Add the balsamic vinegar. Cover the pan and let the onions caramelize for 10 minutes over low heat. Season with salt and pepper.
4- Slice the goat cheese into thick slices (be sure to remove the straw from the center of the cheese if necessary).
5- Remove the onions from the heat. Fill each tartlet with some caramelized onions. Top with a slice of goat cheese and bake for 10 minutes.
6- Remove from the oven and serve immediately.