Put the potatoes into a large pot, with the larger ones on the bottom, and add water to come halfway up the potatoes. Cover the pot and bring to a boil over high heat. When the water begins to boil, carefully drain off about half of it, then return the pot to the heat, cover it again, reduce the heat to low, and let the potatoes steam for about 40 minutes. Turn off the heat; cover the potatoes with a clean, damp tea towel; and let sit for 5 minutes more.

Meanwhile, bring the milk to a simmer in a small saucepan, then add the scallions and simmer for 10 to 15 minutes or until soft. Strain the milk, reserving the scallions and milk separately.

Drain and carefully peel the potatoes, then return them to the pot and mash them well. Stir in the scallions and butter, then drizzle in the milk, continuing to mash, until the potatoes are fluffy. Season to taste with salt and pepper.

Pea Champ: Pea Champ is a specialty of Ireland’s northern counties: Prepare Champ as directed, substituting 1½ cups shelled fresh peas (about 1½ lb/750 g peas in pods) for the scallions, and mashing them into the potatoes.