Start the water and tamari mixture in a large soup pot. Add the onion, celery, mushrooms and barley. Bring this to a boil. In a separate bowl, mix the thicker. Use a whisk and be sure to eliminate all lumps. Set aside. Once the soup pot comes to a boil and the barley is tender, add the thickener slowly while stirring the soup. Let it simmer, stirring as to not burn the soup, for about 15 minutes.