Graciously given to me by my husband's Great Aunt Dorothy, this sour soup is traditionally served as part of the Sweet and Sour Christmas Eve celebration. It's a uniquely flavored soup that's quite simple to make.
In a 3-quart saucepan, saute mushrooms in butter for about 5 minutes, stirring occasionally. Season mushrooms with pepper when the mushrooms first go into the pan. Wait a few minutes before adding the salt as this will draw moisture out and prevent them from coloring.
Add water to pot, making sure there's about an inch of room at the top.
Bring to a low boil and cook for about 30 to 45 minutes.
Stir in rinsed sauerkraut.
In a separate saute pan over medium/medium low heat, add oil and onion, cooking for a minute or so. Sprinkle flour over oil/onion mix to form a roux. Cook for a few minutes until a light blond.
Gradually add a cup of liquid from the soup pot into the roux mixture. Allow it to fully incorporate.
Gradually add the roux mixture into the soup pot, whisking to avoid lumps.
Add white vinegar to soup. If you're not sure about how sour you'd like it, add 1 Tablespoon at a time, tasting between each addition.
About 30 minutes before serving the soup, stir in cooked rice.