Roughly chop sauerkraut. If you are worried about it being too sour you can rinse it, but I don't.
Add sauerkraut to onions and stir. Add enough water to cover the sauerkraut by an inch or two. Add a sprinkling of caraway seeds and two bay leaves. Let simmer until soft, at least an hour.
Soak dry mushrooms in cold water.
Cover prunes in a separate small pot and bring to a simmer until plump, just a few minutes.
Add mushrooms to pot and cook for 5 minutes. Add prunes. If you want to add cream, do so now.
Mix arrowroot (or flour or cornstarch) with a little water and pour into the pot, stirring as you pour.
Serve garnished with sour cream.
Recipe by Almost Bananas at https://www.almostbananas.net/vianocna-kapustnica-slovak-christmas-sauerkraut-soup-vegetarian-version/