Rinse the Cranberries and pick out any stems or soft berries.
Place the cranberries in a saucepan with enough Water to just cover them and add the Sugar to taste.
Bring the mixture to a gentle boil, then reduce the heat and simmer until the cranberries have burst and released their juices, about 8–10 minutes.
If you prefer a smooth kissel, pass the cooked fruit through a sieve or blend briefly and strain to remove skins and seeds; otherwise leave as is for a rustic texture.
In a small bowl, dissolve the Potato Starch in a little cold Water to make a smooth slurry, then slowly whisk this into the hot cranberry liquid over low heat.
Continue stirring until the kissel thickens and becomes glossy (1–2 minutes); remove from heat and let cool slightly before serving warm, or chill until set. Adjust sweetness if needed and serve with cream, yogurt, or fresh fruit.
