In a large kettle cover the potatoes with 2 inches salted cold water, bring the water to a boil, and simmer the potatoes for 40 to 50 minutes, or until they are tender. Drain the potatoes and force them through a ricer or peel them and mash them with a potato masher. Stir in the butter and enough of the milk, heated, to reach the desired consistency. Stir in the scallion and salt and pepper to taste.