Preheat oven to 425°. Place cauliflower on a 15x10x1-in. baking sheet. Melt 1 tablespoon butter; drizzle over cauliflower. Add oil; toss to coat. Roast until lightly browned and tender, 20-25 minutes., In a large saucepan, melt cream cheese and remaining 3 tablespoons butter over medium heat. Add 2 cups cheddar, 1 cup mozzarella and milk. Cook and stir until sauce begins to thicken, 1-2 minutes. Gently fold in cauliflower. Transfer to a greased 8-in. square baking dish. Sprinkle with remaining 1-1/2 cups cheddar and 1/2 cup mozzarella., Bake, uncovered, until bubbly and cheese is melted, about 20 minutes. Let stand 10 minutes before serving.