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 the thinly sliced onions and sea salt. Cook, stirring frequently, for 15 to 20 minutes until the onions are soft and deeply golden. Stir in the brown sugar and continue cooking for another 5 minutes until richly caramelized. Season with black pepper and fresh thyme leaves if using. Remove from heat and set aside.
03 - While the onions cook, toast the 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 prepared parchment-lined baking sheet. If using puff pastry, wrap the brie completely with the pastry sheet, sealing the edges and brushing the surface with beaten egg wash.
05 - Spoon the caramelized onions evenly over the top of the brie. Sprinkle with half of the toasted chopped pecans.
06 - Bake for 15 to 18 minutes if unwrapped, or 20 to 25 minutes if wrapped in puff pastry, until the cheese is soft and gooey throughout.
07 - Remove from the oven and top with the remaining toasted pecans. Allow the brie to rest for 5 minutes before serving so the cheese settles slightly.
08 - Serve warm alongside crackers or sliced baguette for spreading.