Cut top 1 1/2" off the tops of half the asperagus and reserve. Slice remaining asparagus.
Melt 2 Tbl of butter in a large saucepan. Gently fry the sliced shallots until soft but not brown, 2-3 minutes. Add the asparagus and fry for about 1 minute before adding flour. Wait an addtional minute before adding stock or water, lemon juice and seasoning to taste. Bring to a boil before reducing to simmer partially covered for 15-20 minutes or until asparagus is exceptionally tender.
Puree soup. In another pan, melt the remaining butter fry the reserved asparagus tips for 3-4 minutes. Bring soup back up to temperature and add cream and asparagus, heating slowly. Garnish with chervil.