Preparation Generously cover potatoes with cold water in a small saucepan. Add 1/2 teaspoon salt. Simmer, covered, until just beginning to get tender, about 8 minutes. Add peas and simmer, partially covered, until potatoes are tender, about 5 minutes. Drain, then return to pan. Add butter, milk, and 1/8 teaspoon each of salt and pepper. Let stand off heat 1 minute, then add scallion. Coarsely mash.
Preparation Generously cover potatoes with cold water in a small saucepan. Add 1/2 teaspoon salt. Simmer, covered, until just beginning to get tender, about 8 minutes. Add peas and simmer, partially covered, until potatoes are tender, about 5 minutes. Drain, then return to pan. Add butter, milk, and 1/8 teaspoon each of salt and pepper. Let stand off heat 1 minute, then add scallion. Coarsely mash.