Place potatoes in a Dutch oven and cover with water. Bring to a boil.
Reduce heat, then cover and simmer 15 minutes or until fork 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 well greased 2-qt. baking dish.
Top with remaining cheese.
Bake, uncovered, at 350° for 30 minutes or until heated through and cheese is melted.