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, salt, and pepper. Cook, stirring frequently, until the onions are soft and deep golden brown, about 18–22 minutes. Stir in the brown sugar and balsamic vinegar if using, and continue cooking for 2–3 more minutes until jammy and glossy. 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 often, until fragrant and lightly browned. Remove from heat and roughly chop.
04 - Remove the brie from any outer packaging, keeping the rind intact. Place the wheel on the prepared parchment-lined baking sheet or in an 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.
06 - Bake for 12–15 minutes, until the brie is soft and melty throughout but not bursting or collapsing.
07 - Carefully transfer the baked brie to a serving platter. Serve immediately with sliced baguette or crackers alongside.