In a large skillet, heat 2 tablespoons of olive oil and 1 tablespoon of butter over medium heat. Add 3 large sliced onions and sauté them, stirring occasionally, for about 15-20 minutes until they soften and turn golden brown. Stir in a pinch of sugar, 1 tablespoon of balsamic vinegar, and a few sprigs of fresh thyme. Cook for an additional 5 minutes, then remove from heat and set aside.

Preheat your oven to 400°F (200°C). In a mixing bowl, combine 4 ounces of softened cream cheese, 1 cup of grated Gruyère cheese, 4 ounces of goat cheese, and 1 beaten egg. Blend until smooth and creamy. Adjust seasoning with salt and pepper.

Roll out your tart crust or puff pastry on a lightly floured surface to about 1/8-inch thick. Cut out circles and press them into greased tart pans. Leave a bit of crust hanging over the edges. Divide the cheese filling among the tart shells, then top with caramelized onions.

Place the assembled tarts in the preheated oven and bake for 20-25 minutes, or until the crust is golden brown and the filling is set. Monitor after 20 minutes.

Once baked, remove the tarts from the oven and cool for 5-10 minutes. Serve warm or at room temperature with optional garnishes.