Broccoli Cauliflower Casserole

Name: Broccoli Cauliflower Casserole
Ingredients: Unsalted butter or cooking spray; 2 lb. cauliflower (about 1 medium head), cut into small florets; 2 c. water, divided; 1 lb. broccoli (about 2 large heads), cut into small florets; 1/2 c. cream cheese, room temperature; 3/4 c. sour cream; 1/2 c. coarsely chopped sun-dried tomatoes; 3 scallions, white and green parts separated, finely chopped; 1 clove garlic, finely chopped; 1 tbsp. cornstarch; 1 (1-oz.) ranch powder packet; 1 tsp. crushed red pepper flakes (or similar spice of your choosing); Kosher salt; Freshly ground black pepper; 2 c. coarsely shredded or torn fresh mozzarella (about 16 oz.), divided; 3/4 c. grated Parmesan (about 3 oz.)