01 - Preheat the oven to 375°F. Line a baking sheet with parchment paper.
02 - Melt butter with 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 softened and deeply golden. Stir in brown sugar and continue cooking for an additional 5 minutes until richly caramelized. Season with black pepper and fresh thyme leaves. 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 and lightly browned. Roughly chop and set aside.
04 - Place the Brie wheel on the parchment-lined baking sheet. If using puff pastry, wrap the wheel completely, sealing the edges, and brush the surface with beaten egg wash.
05 - Spoon the caramelized onions over the top of the Brie and scatter half of the toasted pecans on top. Bake for 15 to 18 minutes for an unwrapped wheel, or 20 to 25 minutes if enclosed in puff 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. Serve warm alongside crackers or sliced baguette.