Place the potatoes and 2 1/2 tablespoons salt in a large pot. Cover with cold water. Bring to a boil. Reduce heat and simmer until tender, 20 to 25 minutes.
Meanwhile, in a saucepan, over medium heat, heat the butter and 1/2 cup of the heavy cream until the butter melts.
Drain the potatoes and return to pot. Add the butter-cream mixture, sour cream, nutmeg, pepper, and the remaining salt. With a handheld potato masher, mash the potatoes. If a thinner consistency is desired, add the remaining cream, 1 tablespoon at a time. Transfer the mashed potatoes to a heatproof serving dish. (You can cover the dish with aluminum foil and place it over a pot of simmering water, stirring occasionally, for up to 2 hours.)
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