Mrs Kostyras Mushroom Barley Soup

Name: Mrs. Kostyra's Mushroom Barley Soup
Ingredients: 2 ounces dried mushrooms, such as Polish borowik or porcini; 2 cups warm water; 6 ounces pearl barley, washed; 10 cups Beef Stock Mrs. Kostyra's Beef Stock; 4 tablespoons unsalted butter; 2 carrots, peeled and finely chopped; 2 stalks celery, finely chopped; 1/2 to 3/4 cup sour cream (optional); 3 tablespoons all-purpose flour (optional); ¼ cup finely chopped fresh dill