Rub lamb with garlic, salt and pepper. Cover bones with aluminum foil so they do not burn. Place lamb in a pan fat side up and spread carrots, onions and celery around. Roast uncovered for 25 to 30 minutes at 350 degrees for medium.
Cover lamb with bread crumb and butter mixture and place under a broiler until the bread crumbs are toasted. Remove lamb (keep warm on a platter) and place roasting pan on a burner and add stock and tomato paste, bay leaf, garlic and rosemary. Bring to a boil and simmer for 10 minutes.
Dissolve cornstarch in cold water and stir into the mixture and blend until smooth. Place racks on a plate with strained sauce over or under them.