Creating Caramelized Onion and Brie Chicken can be simple if you follow these steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Caramelize Onions: In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the sliced onions and sprinkle with salt. Cook for about 15-20 minutes, stirring occasionally, until the onions are golden brown and caramelized.
3. Season Chicken: While the onions cook, season the chicken breasts with salt and pepper on both sides.
4. Sear Chicken: In a separate pan, sear the chicken breasts on medium-high heat for about 4-5 minutes on each side until browned. Ensure the center is not fully cooked as it will finish cooking in the oven.
5. Combine Chicken and Onions: Once the chicken is seared, place it in a baking dish. Top each chicken breast with the caramelized onions and then arrange slices of brie on top.
6. Drizzle with Balsamic: Sprinkle the balsamic vinegar over the entire dish for an added layer of flavor.
7. Bake: Place the baking dish in the preheated oven and bake for about 20 minutes or until chicken reaches an internal temperature of 165°F (75°C) and the cheese is melted.
8. Garnish: Remove from the oven and allow it to rest for a few minutes. Garnish with fresh thyme or parsley if desired.
9. Serve: Plate the chicken and ensure to spoon some caramelized onions and melted brie over each piece.

Follow these steps to create a fantastic Caramelized Onion and Brie Chicken that is sure to wow your dinner guests!