After washing and peeling the Potatoes, put them into a saucepan with the onions and add about two quarts of cold water. Bring to a boil and allow to cook about forty minutes. When the vegetables are very tender, mash and pass all through a sieve, and return to the fire. Add the seasoning and butter. Bring to a boil, and add the cream and a beaten egg, serving immediately with Croutons.