01 - Preheat oven to 375°F (190°C). 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 15–20 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 3–4 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 brie completely with the pastry sheet, seal the edges, and brush the surface with beaten egg wash.
05 - Spoon the caramelized onions over the top of the brie wheel. Scatter half of the toasted, chopped pecans over the onions.
06 - Bake for 15–18 minutes (20–25 minutes if wrapped in puff pastry), until the cheese is soft, gooey, and beginning to ooze.
07 - Remove from the oven and top with the remaining toasted pecans. Let the brie rest for 5 minutes to settle before serving.
08 - Serve warm alongside crackers or sliced baguette for spreading.