Heat the oven to 425 degrees. Coat a large rimmed baking sheet with nonstick cooking spray. Combine the olive oil and garlic in a small cup.
Cut the cauliflower lengthwise through the core into 4 (1- to 1 1/4-inch) thick slices (reserve leftover cauliflower florets for another use). Place the cauliflower on a baking sheet and brush both sides with the olive oil mixture. Sprinkle with the rosemary, ½ teaspoon salt and ¼ teaspoon pepper.
Bake for 30 minutes. Carefully turn the cauliflower steaks and continue baking an additional 10 minutes or until golden brown and tender.
Meanwhile, combine sour cream, Parmesan, mayonnaise, lemon juice, 1/4 teaspoon pepper, and 1/8 teaspoon salt in a small bowl.
Place the cauliflower steaks on a serving platter and drizzle with the sauce. Sprinkle with the pomegranate seeds. Pass any remaining sauce.