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 butter. Add sliced onions, salt, and pepper. Cook, stirring frequently, until onions are soft and deep golden brown, about 18–22 minutes. Stir in brown sugar and balsamic vinegar if using, and continue cooking 2–3 more minutes. Set aside to cool slightly.
03 - While the onions cook, toast pecans in a dry skillet over medium heat for 3–4 minutes, stirring often, until fragrant. Remove from heat and roughly chop.
04 - Remove brie from any outer packaging and place on the parchment-lined baking sheet or in a small ovenproof dish.
05 - Spoon caramelized onions over the brie wheel, then sprinkle toasted pecans and fresh thyme evenly across the top.
06 - Bake for 12–15 minutes, until the brie is soft and gooey throughout but not bursting or losing its shape.
07 - Transfer baked brie to a serving platter. Serve immediately with sliced baguette or crackers alongside.