Place potatoes in a large saucepan; cover with water. Cover and bring to a boil. Cook for 20-25 minutes or until very tender; drain well., In a bowl, mash potatoes. Add cream cheese, cheddar cheese, sour cream, onion, egg, salt and pepper; beat until fluffy. Transfer to a greased 2-qt. baking dish. Cover and bake at 350° for 40-45 minutes or until heated through. Sprinkle with additional cheese if desired.