Bring the raisins to the boil with the lemon zest in the water.
Mix the pudding powder in a little water according to the instructions and thicken the water with it. Add the lemon juice and mix with the egg yolk. Add sugar to taste as desired.
Beat the egg white until stiff and place on top of the soup as dumplings.
Remove the pot from the heat and close the lid so that the foam dumplings are warm.