In a large saucepan, cook the fiddleheads, mushrooms, leeks and onions in butter until the onions are tender, about 4 minutes.
Stir in the flour, salt and cayenne. Gradually add the milk and broth, stirring until blended. Bring to a gentle boil; cook and stir for 10 minutes or until thickened. Reduce heat; simmer, uncovered, for 10 minutes or until heated through. Stir in lemon juice. Yield: 4 servings.