Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Using a sharp knife, carefully cut the sourdough loaf into a crosshatch pattern, making deep cuts but not slicing all the way through the bottom.
Place the bread on the prepared baking sheet. Gently pull apart the cuts to create space for the filling.
In a mixing bowl, combine the cranberry sauce, Brie cheese, and chopped rosemary. Season with salt and pepper.
Spoon the cranberry and cheese mixture generously into the cuts of the bread, making sure to fill each section.
Brush the melted butter all over the top and sides of the bread.
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and the bread is golden brown.
Remove from the oven, garnish with additional rosemary if desired, and serve warm.
