Put the potatoes in a pan and cover with cold water. Place over medium heat and bring to the boil. Simmer until fork-tender, 20 to 25 minutes, then drain.
Smash the potatoes with a potato smasher for a minute or so to break them down a little. Add the butter, sour cream and heavy cream, then mix thoroughly. Stir in the bacon, green onions and grated cheese. Season to taste. Serve hot, topped with some extra green onions for garnish.