Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Toast the walnuts in a dry skillet over medium heat for 4-5 minutes, until fragrant. Set aside.
Place the brie on the prepared sheet and score the top rind in a crosshatch pattern. (Or wrap the brie in puff pastry with some of the honey and walnuts, seal it, and brush with the beaten egg.)
Bake the plain brie for 12-15 minutes (or the pastry-wrapped brie for 20-25 minutes), until the center feels soft.
Drizzle with honey, scatter the toasted walnuts, and sprinkle with rosemary and flaky sea salt. Serve immediately with crackers, bread, and apple slices.