01 - Preheat oven to 350°F (180°C).
02 - Heat butter and olive oil in a skillet over medium-low heat. Add thinly sliced onions and cook for 15–20 minutes, stirring frequently, until golden and very soft.
03 - Stir in brown sugar, thyme leaves, salt, and black pepper. Continue cooking for 2–3 minutes until onions are deeply caramelized. Remove from heat and set aside.
04 - In a dry skillet over medium heat, toast pecans for 3–5 minutes until fragrant, shaking the pan occasionally. Chop coarsely if desired.
05 - Place the Brie wheel in a small baking dish. For a wrapped version, lay thawed puff pastry flat, set the Brie on top, layer with onions and pecans, fold pastry around the cheese, and brush with beaten egg.
06 - Spoon the caramelized onions over the Brie and scatter 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 crust is golden brown.
08 - Let stand for 5 minutes to allow the cheese to settle. Serve warm alongside crackers or toasted baguette slices.