01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - Melt butter with 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 brown sugar, thyme leaves, salt, and pepper into the onions. Continue cooking for 2–3 additional minutes until 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. Coarsely chop if desired.
05 - Place the brie wheel in a small ovenproof baking dish. For a wrapped variation, 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 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 deep golden brown.
08 - Allow the brie to rest for 5 minutes before serving. Present warm alongside crackers or toasted baguette slices.