Toss these for a few minutes in the butter, then add two glasses of dry white wine and hot water in sufficient quantity, allowing for reduction; also a leek cut in pieces, a clove of garlic, salt and pepper, and a bouquet of thyme, bayleaf, parsley and celery. Bring to the boil and let simmer for two hours. After that pass the soup through a fine strainer, put it back in the saucepan and add a liaison of yolk of egg diluted with fresh cream. For the quantities above two yolks and two tablespoonfuls of cream would do. At the last minute add two or three pieces of fresh butter and serve with croûtons.