Preheat oven to 375º F. Slice off the top of the brie and place cheese in a small baking dish, such as a pie or gratin dish. Place dish in oven and bake until melty, about 7 minutes.
Remove dish and top brie generously with cranberry sauce and honey (if desired). Stir to combine. Place rosemary sprig on top and bake another 10-15 minutes until melted and bubbly. Serve warm with crusty bread or crackers for dipping.
How to make crostini for dipping: Cut a baguette or French or Italian loaf of bread into slices about a quarter of an inch thick. Brush both sides of the slices with olive oil and bake for about 10 minutes at 375º, flipping over halfway through the cook time.