6 pounds red potatoes, scrubbed and cut into 2-inch chunks
2 sticks (1/2 pound) unsalted butter, softened
2 cups warmed milk
2 scallions, thinly sliced
Freshly ground pepper
Put the potatoes in a large pot and cover with water. Cover the pot and bring to a boil. Add a large pinch of salt and boil, uncovered, over moderately high heat, until fork tender, about 25 minutes. Drain the potatoes and return to the pot.
Shake the pot over moderately high heat to dry the potatoes. Off the heat, lightly mash the potatoes. Add the butter and milk and mash until incorporated. Stir in the scallions, season with salt and pepper and serve.
The mashed potatoes can be made early in the day. Cover and keep warm, then reheat gently, adding more warm milk if the potatoes seem stiff.