If you cannot find the fresh oyster mushrooms, substitute 2 ounces of dried oyster or maitake mushrooms. Soak the mushrooms in enough hot water to cover for a half hour. Strain through a filter. Reserve the water. Coarsely chop the dried mushrooms.
Melt the butter in a stockpot and saute the onion, celery, 2 tablespoons of the parsley, carrot, garlic, and fresh mushrooms until soft, about 5 minutes.
Lower the heat and add the flour, stirring every 30 seconds for about 5 minutes or until thick. In a soup pot heat the chicken broth. Add a cup of mushroom mixture at a time to the pot, stirring.
Turn the heat to high, and add the reserved mushroom water and barley. Stir well and add salt to taste.
Simmer, covered, for about an hour or until the barley is tender and the soup is thickened, stirring often. Add additional chopped parsley, mix thoroughly, and adjust seasonings. Enjoy!