Place potatoes in a large pot and fill with enough water to cover. Bring to a boil and cook until tender, about 20 minutes.
Drain well. Return to very low heat and allow the potatoes to dry out for a few minutes.
Meanwhile, heat the milk and spring onions gently in a saucepan until warm.
Mash the potatoes, salt and butter together until smooth. Stir in the milk and onion until evenly mixed. Season with freshly ground black pepper. Serve piping hot in bowls. Set out some extra butter for individuals to add to their servings.