01 - Preheat oven to 350°F.
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 golden brown, about 18–22 minutes. Stir in the brown sugar and balsamic vinegar, if using. Continue cooking for 2–3 more minutes, then 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 a parchment-lined baking sheet or in a small 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 the brie for 12–15 minutes, until the cheese is soft and melty throughout but not bursting or oozing beyond its rind.
07 - Transfer the baked brie to a serving platter. Serve immediately with sliced baguette or an assortment of crackers.