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 mixing 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 degrees for 40-45 minutes or until heated through.