Sorrel soup (Щавелевый суп) is very common in Slavic roots. My family had plenty of it growing up. My husband, however, doesn't like sour food, so I haven't made much of it at home.
Funny thing, while we were in Europe, the last few weeks, our boys stayed with my family and there they were introduced to the sorrel soup. Since then, they refused to eat anything else. My mother said that all they wanted to eat, was kasha and sorrel soup.
Right before we came back to US, my mother promised to make them this soup one last time. Well, she ended up not making it, and Jacob, my 6 year old, complained all the way home, when I picked him up, that grandma forgot to make this soup for them. So, this was a first thing I cooked when I got home. Oh, you should see their joy and excitement, I still can't believe that they love it so much.
This is my mom's recipe, she is a pro at it. Enjoy!