01 - Preheat oven to 375°F. Line a baking sheet with parchment paper and set aside.
02 - In a large skillet, melt butter with olive oil over medium-low heat. Add thinly sliced onions and sea salt, stirring frequently. Cook for 15–20 minutes until onions are soft and deeply golden. Stir in brown sugar and continue cooking for another 5 minutes until richly caramelized. Season with black pepper and fresh thyme leaves. Remove from heat and let cool slightly.
03 - While the onions cook, toast 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, sealing the edges. Brush the pastry with beaten egg wash for a golden finish.
05 - Spoon the caramelized onions over the top of the brie and sprinkle with half of the toasted pecans. Bake for 15–18 minutes (20–25 minutes if wrapped 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. Serve warm alongside crackers or sliced baguette.