Measure the water and remove some of it to mix the cornstarch.
Put the measured water in a saucepan. Add the sugar and fruits to the saucepan as well. Bring to the boil and cook for about 30 seconds.
In the meantime, mix the cornstarch with the removed water. Chop the fruits with the hand blender, stir in the starch mentioned and bring to the boil again briefly.
Mix the fruit puree with the natural yoghurt and allow to cool. When it has cooled a little, put it in the refrigerator.
I always prepare natural yoghurt myself in a yoghurt maker that is designed for 1 liter of yoghurt.