Wash and peel the asparagus and cut off the woody ends. Cut them into pieces of about 4 cm length. Cook the asparagus in 1 1/2 litres of boiling salted water for about 15 minutes and remove with a skimmer.
Heat the fat in a saucepan, add the flour and sauté. Stir in the asparagus water bit by bit. Bring to the boil, add asparagus and peas. Simmer the soup over a mild heat for about 5 minutes. Season to taste with salt, pepper, sugar and lemon juice.
Whisk cream and egg yolk and stir into the soup. Do not let it boil anymore. Pluck chervil leaves from the stems and sprinkle over the soup.