Melt the butter in a saucepan, and sweat the leeks and onion, stirring occasionally, for about 5 minutes. Add the potatoes, stock, lemon juice, nutmeg, coriander and bay leaf. Season with salt and pepper. Bring to the boil, cover and simmer for 30 minutes until the vegetables are soft.
Cool the soup a little, remove the bay leaf and purée the soup in a blender or food processor until smooth. Pour into a clean pan.
Blend the egg yolk into the cream, add a little of the soup to the mixture, then whisk it all back into the soup and reheat gently. Cool and chill before serving.