How to Make It
Place the potatoes in a large pot and add 2 teaspoons salt and enough cold water to cover. Bring to a boil. Reduce heat and simmer until the potatoes are very tender, 20 to 25 minutes. Add the peas, cook for 30 seconds more, then drain and return the potatoes and peas to the pot.
Add the sour cream, butter, 1 teaspoon salt, and ¼ teaspoon pepper to the potatoes and mash to the desired consistency. Fold in the scallions.