In large bowl, combine cauliflower and Brussels sprouts. Drizzle oil on top. Add garlic, rosemary and pepper and toss well. Cover tightly and refrigerate overnight.
Preheat oven to 450°F. Spread vegetables in single layer on large baking sheet with sides. Sprinkle with salt. Roast until vegetables are crisp-tender and beginning to brown at edges, 15 to 20 minutes. Serve hot or at room temperature.