Place potatoes in a Dutch oven and cover with water; bring to a boil. Reduce heat; cover and simmer until tender. Drain and place in a 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° for 30 minutes or until heated through and cheese is melted.