In a large saucepan, saute onion in butter 3-4 minutes or until soft. Stir in flour to form a smooth paste. Cook for 2 minutes. Gradually stir in broth and bring to a boil. Add asparagus and watercress; cover and simmer 5-7 minutes or until vegetables are tender. Cool.
In a blender, process soup in batches until smooth. Return all to pan; stir in cream. Heat on low until cooked through (do not boil). Season with the salt, white pepper and cayenne. Serve with a dollop of sour cream.