This type of cold soup thickened with starch is called kiisseli in Finnish. Kiisseli is usually made from berries and fruits, but you can also make it from milk.
3. Make a slurry from the potato starch and water and pour it into the pan while mixing simultaneously.
4. Wait for the soup to bubble. Then you can lift it from the stove and let cool down.
If you make a pureed thicker version, it is nice to serve with vanilla ice cream, frozen yogurt, granola, yogurt, on pancakes, or even with cheesecake.