Preheat oven to 425°F then arrange the cauliflower in one-layer on a rimmed baking-sheet and brush both sides with oil. Season with pepper and salt.
Roast, turning once till cauliflower is golden and tender, for 35minutes. Then remove from the oven & top each cauliflower-steak with marinara-sauce.
Sprinkle with mozzarella and half of Parmesan. Switch oven to broil and broil cauliflower till cheese is bubbly and golden in spots, for 3minutes.
Serve remaining Parmesan, red pepper flakes and basil leaves.