01 - Preheat your oven to 350°F (180°C).
02 - Heat the butter and olive oil in a skillet over medium-low heat. Add the sliced onions and cook for 15–20 minutes, stirring frequently, until they turn golden and become very soft.
03 - Stir in the brown sugar, thyme leaves, salt, and pepper. Continue cooking for 2–3 additional minutes until the onions are deeply caramelized. Remove from heat and set aside.
04 - In a dry skillet over medium heat, toast the 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 pastry-wrapped version, lay the brie on a sheet of thawed puff pastry, top with the caramelized onions and pecans, fold the pastry around the cheese, and brush with beaten egg.
06 - Spoon the caramelized onions over the brie wheel and scatter the toasted pecans on top.
07 - Bake for 15–18 minutes, or until the cheese is soft and beginning to ooze. If using puff pastry, bake for 20–25 minutes until the pastry is golden brown.
08 - Allow the brie to stand for 5 minutes before serving. Serve warm alongside crackers or toasted baguette slices.