01 - Preheat 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 deeply golden and very soft.
03 - Stir in brown sugar, thyme leaves, salt, and black pepper. Continue cooking for 2–3 additional minutes until the onions are fully caramelized and jammy. 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. Coarsely chop if desired.
05 - Place the brie wheel in a small baking dish. For a wrapped version, lay thawed puff pastry flat, set the brie in the center, top with caramelized onions and pecans, fold the pastry around the cheese, and brush with beaten egg.
06 - Spoon the caramelized onions evenly 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 crust is golden brown.
08 - Allow the brie to rest for 5 minutes before serving. Serve warm alongside crackers or toasted baguette slices.