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).
Put an onion or two in the pan and 2 sprigs of fresh rosemary. Place the roasting pan in the oven set at 350F(190C) for 15-20 minutes per pound (you don't want to cook lamb well done, but more medium to medium rare) a 6 pound leg should take about 1 1/2 to 2 hours.Remove the cover(or foil) for the last 15 minutes of roasting time.ut an onion or two in the pan and 2 sprigs of fresh rosemary. Place the roasting pan in the oven set at 350F(190C) for 15-20 minutes per pound (you don't want to cook lamb well done, but more medium to medium rare) a 6 pound leg should take about 1 1/2 to 2 hours.