Micha, I got this recipe from a Milk Calendar several years agoit has been a favorite ever since.

Leek & Mushroom Soup

1/2 cup butter
2 leeks, washed well, white part only, sliced
1/2 pound mushrooms, chopped
1/4 cup flour
1 teaspoon salt
dash cayenne pepper
1 cup chicken broth
3 cups milk
1 tablespoon Sherry (or lemon juice)
salt and pepper to taste

In 1/4 cup of butter, saute leeks until tender but not brown. Remove & set aside. In remaining butter, saute mushrooms until soft but not brown. Blend in flour, salt, & cayenne. Gradually stir in broth and milk. Cook, stirring until mixture thickens & comes to a boil. Add leeks, sherry, salt & pepper to taste. Simmer 10 minutes. Serve with thin slices of lemon & a sprinkle of parsley.