You will definitely need:
- sour cream (or very thick, creamy yoghurt; don't bother with the thinner kinds)
You may also want to use:
- caraway seeds or other spice to your preference (I like my food mild, so I prefer caraway)
- prunes / dried plums
I cannot give you exact amounts; it all depends on how much of the soup you want to make and how rich do you like your soups. I'd recommend using at least a cup of the sour cream in either case, though.
You start by cubing your potatoes and then cooking them and the sauerkraut (both salted) separately in two pots. Then you unite them in one pot, add water to the amount you want to have, add the sour cream and cook a bit more. And that's it. It tastes better after a day or two when the tastes "settle", but you can eat it fresh from the stove as well.
- You can mix a tablespoon of flour into the sour cream for more thickness - I'd recommend doing this if you use yoghurt, with sour cream it's not necessary.
- Add cubed sausage.
- Add cubed prunes as well (they expand a lot in the soup, so that's why cubed). They lose a lot of their taste themselves, but add a nice overtone to the whole soup.
A variant with a not very spicy sausage and prunes. The prunes make it darker than the plain variant.
A variant with spicy Hungarian sausage and prunes. Paprika!
And, really, that's it. I like it when the sauerkraut is very sour, but that's my own personal preference. The fun thing is, it's so versatile, depending on the type of sausage and/or spice you use; while preparing for this belated post, I ate sauerkraut soup for about three weeks in a row, and did not get bored with it, because I used different kinds of everything.
EDIT Oct 24 2014: I linked to this on Red and Honey's soup link party. Because once again, I'd lived on this soup and did not get bored with it. And the more people know about its simple awesomeness, the better.
(There could even be a link button, but for some reason, it does not work for me. To see the link button that did not show, follow the link above. :D)