Remove the fell (tissue-like covering) from lamb with a sharp knife. Make several small slits on outside of lamb, and stuff with garlic slices. Sprinkle with 1/2 teaspoon pepper. Place lamb, fat side up, on a rack in a large roasting pan.
Bake at 325° for 2 1/2 hours. Add potatoes, remaining pepper, and rosemary; bake 1 1/2 hours or until meat thermometer registers 175°. Remove lamb and potatoes to a warm platter; reserve pan drippings.
Sauté parsley and celery in butter in a small saucepan until celery is tender. Add remaining ingredients; cover and simmer 5 minutes. Skim fat from reserved pan drippings. Combine parsley mixture and pan drippings; cook over low heat until heated. Serve with lamb and potatoes.