Preheat oven to 415 degrees. Cut the cauliflower away from the core and rinse. Pat dry, then slice or break into small pieces. Halve the grapes and mince the garlic.
Add all to a large baking sheet. Sprinkle sea salt, black pepper. Mince one sprig of rosemary, add to pan. Drizzle olive oil on top of all, then toss to coat, directly on the pan. Spread vegetables out, so they are barely touching, if at all.
Roast in oven for 20 minutes, then stir to ensure both sides will get that lovely golden brown crust. Roast for another 20 minutes. Take out of oven. Chop remaining sprig of rosemary, and sprinkle on top, then stir. Serve.