01 - Preheat oven to 350°F.
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 deeply golden brown, about 18–22 minutes. Stir in the brown sugar and balsamic vinegar if using. Continue cooking for 2–3 more minutes until the mixture is glossy and jammy. 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 and place it on a parchment-lined baking sheet or in a small ovenproof dish.
05 - Spoon the caramelized onions over the top of the brie wheel. Scatter the toasted pecans and fresh thyme leaves evenly over the onions.
06 - Bake for 12–15 minutes, until the brie is soft and melted throughout but still holding its shape and not bursting.
07 - Transfer the baked brie to a serving platter and serve immediately with sliced baguette or crackers alongside.