Wash leeks very well; slice and use white part only. In ¼ cup butter, sauté leeks until tender but not brown. Remove and set aside. In remaining butter, sauté mushrooms until soft,
about 10 minutes. Blend in flour, salt and cayenne. Gradually stir in broth and milk. Cook, stirring, until mixture thickens and comes to a boil. Add
leeks, sherry, salt and pepper to taste. Simmer for 10 minutes. Serve with thin slices of lemon and sprinkling of parsley, if desired.