Preheat oven to 400 F (205C) and line 3 baking sheets with parchment paper. To make crostini, in a medium bowl, toss baguette slices with olive oil and rosemary. Place on baking sheet and bake until slightly golden and crisp, turning once for even baking on both sides, about 7-10 minutes. Remove from oven and let stand until ready to serve.
Toss the grapes with olive oil, a little salt and pepper and place in the oven. Roast for 15-20 minutes, until the grapes are wrinkly and have a few dark, caramelized spots (especially on the bottom). Leave the oven on.
Warm brie for 10-15 minutes in the same oven. Test by pushing on the center of the brie.
Top brie with grapes, balsamic reduction, and more honey, if desired. Garnish with rosemary.
