01 - Preheat oven to 350°F. Line a small baking dish with parchment paper if desired.
02 - Melt butter with olive oil in a skillet over medium-low heat. Add thinly sliced onions and cook for 15–20 minutes, stirring frequently, until deeply golden and very soft.
03 - Stir brown sugar, thyme leaves, salt, and pepper into the onions. Continue cooking for 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 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 pastry-wrapped version, lay the brie on a sheet of thawed puff pastry, top with onions and pecans, fold the pastry around the cheese, and brush the surface with beaten egg.
06 - Spoon the caramelized onions over the brie and scatter toasted pecans on top.
07 - Bake 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 deep golden brown.
08 - Let stand for 5 minutes to allow the cheese to settle. Serve warm alongside crackers or toasted baguette slices.