In large soup pot melt butter and sweat (cook over low heat to bring out moisture) vegetables till soft but not brown. Add flour and stir with wooden spoon until fully incorporated. Add warm chicken stock, whisking to avoid lumps. Let simmer 15- 20 minutes. Add cream. Cook 15 minutes over medium heat. Stir to avoid scorching. Add crab and sherry. Simmer 10 minutes. Enjoy with warm bread.