Preheat oven to 350°F. Line a baking sheet with parchment paper.
Let the brie sit at room temperature for about 15 minutes, then slice thinly.
Using a serrated knife, slice each croissant horizontally while leaving one side attached.
Add a thin layer of brie to the bottom half of each croissant.
Spread about 2 teaspoons preserves onto each croissant, keeping the preserves centered instead of spreading fully to the edges.
Top with another light layer of brie.
Brush the tops of the croissants lightly with melted butter.
Bake for 8–9 minutes until the croissants are crisp, golden brown, and the brie is softened and creamy.
Let rest for 1–2 minutes before serving.
Finish with flaky sea salt and optional honey or thyme if desired.