Peel potatoes. Place potatoes in a large pot and cover with cold water. Bring to a boil and simmer for 18 to 22 minutes, until potatoes are tender.
In a saucepan combine cream and butter. Heat until very warm. Add salt and pepper. Drain potatoes and quickly pass through a food mill or potato ricer into a large bowl. Whisk cream mixture into the hot potatoes. Add the horseradish and green onions. Serve hot.