01 - Preheat the oven to 350°F (180°C).
02 - Heat butter and olive oil in a skillet over medium-low heat. Add the thinly sliced onions and cook for 15–20 minutes, stirring frequently, until they turn golden brown and become very soft.
03 - Stir in the brown sugar, fresh thyme leaves, salt, and black pepper. Continue cooking for another 2–3 minutes until the onions are fully caramelized and glossy. Remove from heat and set aside.
04 - In a dry skillet over medium heat, toast the pecans for 3–5 minutes until fragrant and lightly browned. Chop coarsely if desired.
05 - Place the brie wheel in a small oven-safe baking dish. For a wrapped variation: lay the brie on a sheet of thawed puff pastry, spoon the caramelized onions and toasted pecans over the top, fold the pastry around the cheese, and brush the surface with beaten egg.
06 - Spoon the caramelized onions over the brie wheel and scatter the toasted pecans on top.
07 - Bake uncovered for 15–18 minutes until the cheese is soft and beginning to ooze. If using puff pastry, bake 20–25 minutes until the pastry is golden brown and puffed.
08 - Allow the baked brie to rest for 5 minutes. Serve warm alongside crackers or toasted baguette slices.