Preheat oven to 425°F. Cut ½ inch off top of garlic head to expose cloves; discard top. Place the garlic head, cut-side up, on a small sheet of foil; drizzle with 1 tablespoon oil. Bring edges of foil together over the garlic and crimp edges to seal; place on a large rimmed baking sheet.

Place cauliflower on a cutting board, stem-side down. Using a large chef’s knife, cut 4 (¾-inch-thick) slices from the center of the head. (Reserve remaining cauliflower for another use.) Arrange the steaks in a single layer on the baking sheet with the garlic. Stir 1 teaspoon garlic-and-herb seasoning and the remaining 1 tablespoon oil in a small bowl until well combined; evenly brush over the tops of the steaks. Roast until tender and browned, 35 to 40 minutes. Remove from oven; gently flip the steaks and carefully open the garlic packet.

Squeeze the garlic cloves out of their skins into a small bowl; using a fork, mash until smooth. Add 4 tablespoons butter, 3 tablespoons Parmesan, ½ teaspoon salt and the remaining ½ teaspoon garlic-and-herb seasoning; stir until smooth. Spread the garlic butter over the steaks (about 2 tablespoons each). Bake until the cheese is melted, about 5 minutes. Sprinkle with herbs.