In a large bowl, combine bread crumbs, garlic, rosemary, 1 teaspoon salt and 1/4 teaspoon pepper. Toss in 2 tablespoons olive oil to moisten mixture. Set aside. Season the rack all over with salt and pepper. Heat 2 tablespoons olive oil in a large heavy oven proof skillet over high heat. Sear rack of lamb for 1 to 2 minutes on all sides. Set aside for a few minutes. Brush rack of lamb with the mustard. Roll in the bread crumb mixture until evenly coated.
For oven cooking, place the lamb in an uncovered roasting pan and cook at 165 degrees C (325 degrees F). For every 0.5 cm (inch) of thickness, allow 10 to 15 minutes of cooking time. You can allow more time for a well-done steak, cooking to an internal temperature of 76 degrees C (170 degrees F).
Slide the broiler pan out of the oven and put the steak on the pan. Put the pan in the oven in the top rack slot. Your broiler pan should sit about four inches away from the flame. London broil only needs four to five minutes of cooking time on each side.Set a timer to help you remember when to flip it. Overcooking renders this cut very tough no matter how long it marinates, so keep track of the time. An eight minute steak will cook to rare while 10 minutes gives you a medium rare steak.ondon broil only needs four to five minutes of cooking time on each side. Set a timer to help you remember when to flip it. Overcooking renders this cut very tough no matter how long it marinates, so keep track of the time. An eight minute steak will cook to rare while 10 minutes gives you a medium rare steak.