Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
Remove the leaves from the cauliflower and trim the stem, keeping the core intact.
Slice the cauliflower into 1-inch thick steaks. Some florets will fall off—save them to roast alongside.
Place the steaks and loose florets on the baking sheet. Brush both sides with olive oil.
Sprinkle with garlic powder, smoked paprika, salt, and pepper.
Roast in the oven for 20 minutes, flipping once halfway through.
Sprinkle mozzarella and Parmesan over the cauliflower, then return to the oven until the cheese is melted and golden, about 10 minutes.
Remove from oven, sprinkle with fresh parsley, and serve warm.