Put the butter and the flour in the saucepan and let them blend without burning. Mix well over a slow fire, and add one pint of Consomme. Add the juice of half a lemon and the chopped parsley, and let all boil about fifteen minutes. When it reaches this point take off the stove and add the yolk of one egg, well beaten; mix well, stirring round, and serve with boiled fish, etc.
Never add egg while the Sauce is on the fire, as it will curdle immediately.