Prepare the Starch Mixture: In a small bowl, combine the cornstarch (or potato starch) with 1 cup of water. Stir it well to create a smooth slurry. Set this aside.

Heat the Fruit Juice: In a medium saucepan, pour the fruit juice and bring it to a gentle boil over medium heat. Add the sugar to the juice and stir until it is fully dissolved.

Thicken the Kisielius: Once the fruit juice is boiling, slowly pour the starch mixture into the juice while stirring continuously. Keep stirring for 2-3 minutes to prevent lumps from forming. Continue to cook for another 5-7 minutes, until the mixture thickens to a pudding-like consistency.

Adjust Consistency: If the Kisielius is too thick, you can add a little more water to achieve your desired consistency. If it’s too thin, mix a small amount of starch with water and add it to the mixture, cooking for a few more minutes until it thickens.

Cool the Kisielius: Once the Kisielius is thickened, remove the saucepan from the heat. Let it cool for about 5 minutes before transferring it to serving bowls.

Chill: Place the bowls in the refrigerator to chill for about 30 minutes or until the Kisielius has fully set.

Serve: Serve the Kisielius chilled, garnished with fresh berries, whipped cream, or a dollop of sour cream for added richness, if desired.