In a medium saucepan cook potatoes, cauliflower, and carrot, covered, in enough boiling salted water to cover for 15 to 20 minutes or until tender. Drain. Mash with a potato masher or beat with an electric mixer on low speed. Add sour cream and salt. Mash or beat until combined. Season to taste with additional salt and black pepper. Top each serving with Parmesan cheese. Makes 6 servings.