In a small bowl, combine ketchup and Worcestershire sauce.
Season meat with salt and pepper; place on rack in open roasting pan. Insert meat thermometer in thickest part of meat, making sure end does not touch bone or rest in fat. Or, use an instant read thermometer while roasting to check temperature.
Roast lamb at 325° for about 25 to 25 minutes per pound, basting occasionally with ketchup and Worcestershire sauce mixture. Meat thermometer should register 145° for rare, 160° for medium, or 170° to 175° for well done. Remove from oven and let stand 10 to 15 minutes before carving.
Allow 1/2 to 3/4 pound per person.
Calories per serving: 546