Preheat the oven to 450 F. Toss cauliflower, 11/2 tablespoons oil, salt, cumin and pepper together in a large bowl and spread in one layer on a large, rimmed baking sheet. Roast 20 minutes and stir the cauliflower.

Toss the grapes and remaining olive oil together and add to the baking sheet. Roast 5-10 minutes longer and serve.