Place oven rack on center level. Heat oven to 500°. Place the racks of lamb in a 14-by-12-by-2-inch roasting pan. Whisk together the oil, mustard, rosemary, salt and pepper in a small bowl. Coat both sides of the racks with the mixture. Pat on a layer of bread or cracker crumbs. Roast 20 minutes for rare. Let stand 5 minutes before carving.
NOTES: (6 to 8 servings) Make sure the butcher removes the chine bone (the backbone) and that he cracks the rack between the ribs so it can easily be cut into chops at the table.