Place potatoes in a pot and cover with water; bring to boil. Reduce heat; cover and simmer until tender. Drain and place in a mixing bowl. Add 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-quart baking dish. Top with remaining cheese. Bake, uncovered, at 350 degrees for 30 minutes or until heated through and cheese is melted.