Place potatoes in a dutch oven and cover with water, bring to 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 onion.
Tranfer to a greased 3-qt. baking dish. Top with remaining cheese and bake,uncovered, at 350 degrees for 30 minutes or until heated through and cheese is melted.
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