Baltic Rhubarb Fruit Soup
500 grams of tender rhubarb stalks
1 litre of water
200 g sugar
2 Tbsp potato starch/potato flour (see Note below)
Rinse the rhubarb stalks and slice thinly (5 mm or 1/5 of an inch). If you're using tender rhubarb stalks, then there's absolutely no need to peel them first.
Place wter and rhubarb into a saucepan and bring to the boil. Simmer for 5-7 minutes, until rhubarb softens.
Add sugar, stir until dissolved.
Mix potato starch with some cold water until you've got a runny paste. Pour this into the fruit soup, stirring constantly. Bring back to the boil and remove IMMEDIATELY from the heat (if cooked for too long, the fruit soup becomes thin again).
Let cool and serve.
Note on the use of starch. It's more common to use potato starch in Baltic countries, but you can certainly use cornflower (UK)/cornstarch (US) instead. If doing so, cook the kissel for a few minutes after adding the starch, as cornflour/cornstarch needs to be heated properly in order to kick-start the thickening action.