Separate asparagus tips from stalks. Boil both in 1 cup water with dash of salt until just tender. Remove and cool in cold water. Drain and set aside. Process asparagus stalks and cooking water in a food processor and strain.
Add broth to asparagus purée, mix and simmer for 15 minutes to reduce
In a separate sauce pan, melt butter and mix with flour. Slowly add 3 to 4 ladle–fulls of asparagus broth to roux to thin it.