1/2 to 1 kg crowberries, well ripened
1 litre water
1 cinnamon stick
2 tbs potato flour or cornstarch
100 ml cold water
100 g sugar, or to taste
Pick over the berries, removing any under-ripe berries and other unwanted objects (may include twig pieces, leaves, moss and spiders - that is assuming you didn't go to Vínberið*** and buy the berries already cleaned) and clean under running cold water.
Cook the berries in the water with the cinnamon stick for 20-30 minutes. Strain and re-heat to boiling. Mix potato flour/cornstarch and water into a paste. When the soup boils, stir in the paste and let boil again. Add sugar to taste. Serve warm with zwieback.
* I tasted Danish crowberries once, and they didn't have nearly as much flavour as the ones that grow in Iceland, and neither were they as juicy. I'm guessing it's the soil that makes the difference.
**you can also make crowberry wine.
***a grocery and confectionery shop in Reykjavík and the only local shop where I have seen crowberries for sale.