Place potatoes in a Dutch oven and cover with water. Bring to a boil. Reduce heat; cover and simmer for 10-15 minutes or until tender. Drain and place in a large bowl. Add the sour cream, milk, butter, salt and pepper. Beat on medium-low speed until light and fluffy. Stir in 2 cups cheese, bacon and onions.
Transfer to a greased 3-qt. baking dish. Top with remaining cheese. Bake, uncovered, at 350° for 30 minutes or until heated through and cheese is melted.