Put the low-fat quark in a bowl and add the oatmeal. It is best to stir vigorously once.
Peel the overripe banana and cut into thin slices. Add this to the low-fat quark and oatmeal. Puree the mixture and only gradually add the water carefully. Take a little more or less water if necessary. Other fruits can also be used.
Pour the shake into a bottle and place in the refrigerator. The shake can be kept for between 24 and 48 hours. Shake well before drinking.