01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Heat butter and olive oil in a large skillet over medium-low heat. Add sliced onions and sea salt, stirring frequently for 15 to 20 minutes until onions are soft and deeply golden. Stir in brown sugar and continue cooking for another 5 minutes until fully caramelized. Season with black pepper and fresh thyme if desired. Remove from heat and set aside.
03 - While the onions caramelize, toast pecan halves in a dry skillet over medium heat for 3 to 4 minutes until fragrant. Roughly chop and set aside.
04 - Place the brie wheel on the parchment-lined baking sheet. If using puff pastry, wrap the brie completely, sealing the edges, and brush the surface with beaten egg wash.
05 - Spoon the caramelized onions over the brie and sprinkle with half of the toasted pecans. Bake for 15 to 18 minutes if unwrapped, or 20 to 25 minutes if enclosed in pastry, until the cheese is soft and gooey throughout.
06 - Remove from the oven and top with the remaining chopped pecans. Allow the brie to rest for 5 minutes before serving warm with crackers or sliced baguette.