Whisked egg yolks and cream thicken a soup slightly but mainly add a rich creamy finish. Do not let the soup boil after you have added eggs or single cream as the eggs will scramble and single cream will curdle. As the egg yolks cook they will thicken the soup. If the soup has been previously thickened with a little flour it will be less vulnerable to curdling but it should still not be boiled.
Allow 75ml (3fl oz) double cream and 2 egg yolks for every 1.2litres (2pints) soup already thickened with flour; increase the number of yolks to 4 if the soup has not been previously thickened.
Although soups can be made well in advance, only thicken them just before serving. To make the thickening mixture put the egg yolks and cream in a medium-sized mixing bowl. Using a fork or whisk, whisk them together lightly until well combined. It is best to use double cream because it will not curdle if the soup is boiled accidentally.
Meanwhile, reheat but do not boil the soup. Add a ladleful of hot soup to the eggs and cream mixture. Use a wooden spoon to stir them together.
Stir the eggs, cream and soup mixture into the pan of hot soup. Continue to stir the soup over a gentle heat until it has thickened slightly but do not allow it to boil. Serve immediately.