In a large pot, bring 4 cups of water to a boil.
Add the berries and sugar to the boiling water. Reduce the heat and let it simmer for about 10-15 minutes, or until the berries are soft and have released their juices.
Remove the pot from the heat and allow the mixture to cool slightly.
Using an immersion blender or regular blender, blend the berry mixture until smooth. If you prefer a smoother texture, you can strain the mixture through a fine sieve to remove any seeds or pulp.
Return the blended berry mixture to the pot and bring it back to a gentle simmer.
In a small bowl, mix the potato starch or cornstarch with 1/4 cup of cold water to create a slurry.
Gradually pour the starch slurry into the simmering berry mixture, stirring constantly to prevent lumps from forming.
Continue to cook and stir until the kisiel thickens to a desired consistency, about 2-3 minutes.
If using, add the lemon juice and stir to combine.
Remove the pot from the heat and let the kisiel cool to room temperature.
Transfer the kisiel to serving dishes and refrigerate until chilled.
Garnish with fresh mint leaves before serving.
Enjoy your meal!