Preheat the oven to 325 degrees F (165 degrees C).
Place potatoes in a large pot of lightly salted water. Bring to a boil, and cook until tender, about 15 minutes. Drain, and mash.
In a large bowl, mix mashed potatoes, cream cheese, sour cream, milk, onion salt, and pepper. Transfer to a large casserole dish.
Cover, and bake for 50 minutes in the preheated oven.
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