Wash and peel the asparagus and cut off the woody ends. Cook the asparagus shells in 800 ml salt water with 1 pinch of sugar for about 20 minutes. Cut the asparagus into pieces. Pour the asparagus water through a sieve, collect and bring to the boil. Cook the asparagus for about 10 minutes. Remove the asparagus. Heat the fat in a pot, sauté the flour in it and deglaze with the asparagus water.
Boil up while stirring. Season with salt, pepper, sugar and lemon juice. Whisk cream and egg yolk and add to the soup. Do not boil any more. Rinse the crab meat. Warm the crab and asparagus in the soup. Arrange on plates and garnish with chervil