Place potatoes in a large saucepan and cover with water. Bring to a boil over high heat. Reduce heat to medium; cover and cook for 15-20 minutes or until tender. Drain., In a large bowl, mash potatoes. Add the sour cream, milk, butter and 1 cup cheese. Stir in the onion, bacon, salt and pepper. Spoon into a greased 2-qt. baking dish. Sprinkle with remaining cheese. , Bake, uncovered, at 350° for 30-35 minutes or until heated though.