Preheat oven to 350 degrees. Place beef, onion, garlic, peppercorns and broth in an oven-proof casserole. Add water to cover by 1 1/2 inches. Bring to a boil. Cover and bake in the oven about 2 hours, until beef is very tender. Remove meat and cool. Shred from bones, discarding any fat and the bones. COver. Pour broth through a fine mesh strainer into a gravy separator or bowl. Allow to cool. Pour or skim off fat and return defatted broth to the casserole. Add leeks, carrots and celery to casserole. Bring to a boil; reduce heat and simmer, covered, 3 minutes. Add shredded beef to broth with mushrooms, pasta, salt and pepper. Bake, covered, for 5 minutes. Remove from oven and stir in dill and parsley. Serve immediately in deep soup bowls.