Steep mushrooms in 1 1/2 cups of boiling water until they are soft and you can chop them (I use a food processor). Save the water to add to the soup. It gives it a nice rich color.
Put sauerkraut, sauerkraut juice, and 1 1/2 quarts water in soup pot (5-quart Dutch oven is fine). Bring to a boil, add chopped mushrooms and mushroom water. Turn down heat and simmer for one hour.
Add barley (make sure you rinse it first), cook until barley is done.
In the meantime, melt the butter and saute onions until they're soft. Add the flour to the onion mixture to thicken. Add some of the boiling liquid from your soup to the onion mixture and when smooth add to soup.
Add salt and pepper to taste. If soup is too thick for your taste, add a little more sauerkraut juice mixed with water. Enjoy.
