On low heat in a heavy saucepan, simmer potatoes in milk until they come to a boil; allow to cook for another 3 minutes, then cover and turn off heat. They will cook through if your pot is heavy and retains heat (e.g., cast iron or enameled cast iron). If you use aluminum or tri-ply, keep simmering at lowest-possible heat until potatoes are cooked through.
When potatoes are done, peel and mash them.
Use the boiled milk to thin and smooth them. Then add scallions, butter, and salt and pepper.