Yes, you can roast frozen cauliflower. Preheat the oven to 450 F (230 C.). Place the frozen cauliflower in a bowl and toss it with lemon juice, garlic powder, salt, and black pepper. Add the cauliflower to a lined baking sheet in a single layer. Bake for 20 -25 minutes, tossing the florets a few times to brown them evenly.