Peel potatoes and cut into large chunks of equal size. Place potatoes in a pot and cover with cold water. Bring to a boil and stir in 1 teaspoon of salt. Reduce heat and simmer until potatoes are tender when stabbed with a fork (start checking at 15 minutes). Add frozen peas to pot and cook for another minute or until peas are heated through.
Drain potatoes and peas; return to pot with milk, butter, 1 teaspoon salt, and freshly ground black pepper, to taste. Mash gently with a handheld potato masher. Cover pot and place back on burner (that has been turned off) to keep warm until ready to serve.