Start by boiling the potatoes until tender, then mash them with sour cream, milk, and butter until you get the right consistency. Sprinkle with chopped scallions for an oniony bite. You can cook this dish up to a day ahead and reheat it in the microwave. Instead of Yukon golds, you can also use red potatoes and leave the skin on to add color and create a more rustic texture. (Red-skinned potatoes and green scallions are ideal if you need an especially festive holiday dish.)
