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, about 18–22 minutes. Stir in the brown sugar and balsamic vinegar if using, and continue cooking for 2–3 more 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 often, until fragrant. Remove from heat and roughly chop.
04 - Remove the brie from any outer packaging and place it 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 bursting out of its rind.
06 - Transfer the baked brie to a serving platter. Serve immediately with sliced baguette or an assortment of crackers alongside.