Wash leeks well; slice and use white part only.
In 1/4 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 thickened and it comes to a boil.
Add leeks and lemon juice.
Simmer for 10 minutes.