1Wash leeks very well; slice and use white and green tender parts only. Sauté leeks in 1/2 cup butter until tender but not brown.
2Add the mushrooms to the leeks and sauté mushrooms until soft, about 10 min. Blend in flour, salt and cayenne. Gradually stir in broth and milk. Cook, stirring, until mixture thickens and comes to a boil. Add additional salt and pepper to taste. Simmer for 10 min.