Heat the rhubarb with the sugar and water in the saucepan, then switch to a low flame and cook until soft for about 10 minutes with the saucepan lid closed.
Let the rhubarb cool down to lukewarm, then put it in the food processor together with the cooking liquid and puree it finely, possibly also strain it through a sieve.
Flavor the puree with rose water and cinnamon, place in the freezer for about 4 hours, stir every 20 minutes with a fork or whisk.