01 - Preheat oven to 350°F. Line a baking sheet with parchment paper or prepare a small ovenproof dish.
02 - In a large skillet over medium heat, melt the butter. Add the sliced onions, sea salt, and black pepper. Cook, stirring frequently, until the onions are soft and deep golden brown, approximately 18–22 minutes. Stir in the brown sugar and balsamic vinegar, if using, and continue cooking for an additional 2–3 minutes. Remove from heat and set aside to cool slightly.
03 - While the onions cook, toast the pecans in a dry skillet over medium heat for 3–4 minutes, stirring frequently, until fragrant and lightly browned. Remove from heat and roughly chop.
04 - Remove the Brie from all outer packaging, leaving the rind intact. Place the wheel on the parchment-lined baking sheet or in the ovenproof dish.
05 - Spoon the caramelized onions evenly over the top of the Brie. Sprinkle the toasted pecans and fresh thyme leaves over the onions. Bake for 12–15 minutes, until the cheese is soft and gooey but not collapsing.
06 - Carefully transfer the baked Brie to a serving platter. Serve immediately with sliced baguette or an assortment of crackers alongside.