Heat oven to 350 degrees. For the crust, combine bread crumbs and butter in a microwave safe bowl on HIGH 1 minute or until light golden brown, stirring once. Add rosemary and garlic to bread crumb mixture; mix thoroughly and set aside.
For the lamb, heat oil in 10-inch sauté pan over medium heat until hot. Season lamb with salt and black pepper. Sear meat on all sides about 1-2 minutes or until well browned. Place lamb on cutting board and spread mustard over top. Press crust onto mustard.
Return lamb to same pan; place pan into oven. Roast until the thermometer registers 140 degrees, about 25-30 minutes. Remove lamb from pan to carving board; let stand, uncovered, at least 10 minutes (internal temperature will reach 145 degrees for medium rare, 160 degrees for medium). Slice into individual chops and serve immediately.
Cook’s Note: A rack of lamb is a section of rib bones with a very tender and juicy strip of meat attached. The bones are "Frenched," which means that the meat and fat are removed from between the ends of the bones for a more elegant presentation.
Searing the lamb on all sides seals in the juices and provides nice browning on the outside surface of the meat.
Larger cuts of meat such as rack of lamb should be removed from the oven when the thermometer registers 5-10 degrees below desired doneness temperature and allowed to stand at least 10-15 minutes. The temperature will continue to rise within the meat and reach the desired doneness.