Place potatoes into 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. (It helps if you place a clean dish towel over the potatoes to absorb any remaining moisture.)
Meanwhile, heat the milk, cream and green onions gently in a saucepan, until warm.
Mash the potatoes, salt and butter together until smooth. Stir in the milk and green onion until evenly mixed. Serve piping hot in bowls. Set out some extra butter for individuals to add to their servings.