Wash and clean the rhubarb and cut into small pieces. Put together with the sugar, vanilla sugar, lemon juice and water in a saucepan and let simmer until the rhubarb collapses.
Stir in the semolina and let it sit a little. Pour the rhubarb and semolina mixture into a metal bowl and beat it in a cold water bath with a hand mixer (mixer) until the mixture has cooled down. This makes the rhubarb semolina mixture wonderfully fluffy and frothy without the addition of cream or egg whites.