Heat oven to 450 F. Cut the stem off the cauliflower and cut remaining head crosswise into four 1-inch thick slices, keeping as intact as possible. Place on a sheet pan and brush both sides with 1 1/2 tablespoons of the olive oil. Sprinkle with rosemary, salt, and 1/4 teaspoon pepper. Roast until browned and tender, about 25 minutes.

While the cauliflower is roasting, combine the remaining olive oil, pepper, grapes, olives, parsley, and pistachios. When cauliflower is roasted, transfer to serving plates and top with the grape mixture. Serve.