Peel the asparagus and cut off the woody ends. Wash the asparagus shells and cook in 1 1/2 litres of salted water for about 20 minutes. Pour through a sieve, collect the asparagus water and bring to the boil.
Cook the asparagus for about 20 minutes. Remove the asparagus and cut into pieces. Heat fat in a pot, sauté flour in it and remove with the asparagus water. Bring to the boil while stirring.
Add asparagus and prawns and season with salt, pepper, sugar, Worcester sauce and lemon juice. Whisk cream and egg yolk and add to the soup. Do not let it boil anymore. Pluck the chervil leaves from the stems and sprinkle over the soup.