Oxtail makes a flavorful soup meat that readily separates from the bones when tender.
Combine stock and oxtail in soup kettle and simmer, partially covered, 2 hours. Skim occasionally during cooking. Remove from heat. Cut meat from bones and chop. Set aside meat and discard bones. Cool stock, chill, and skim fat from top. Return meat and stock to kettle. Heat to simmer. Add barley and cook 15 minutes. As barley cooks, melt butter in small skillet and saute carrots, celery, onion, and mushrooms 5 minutes. Add to soup and season with salt and pepper. Simmer 10 minutes longer. Garnish with chopped parsley.