Trim fat from bones and roast. Sprinkle generously with salt and pepper.
In a food processor or blender, combine cornflakes, panko, and rosemary. Cover and process or blend until finely chopped. In a small bowl, stir together crumb mixture and melted butter; set aside.
For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above the drip pan. Place the lamb roast, bone side down, on the grill rack over the pan. Cover and grill to desired doneness. Allow 50 to 60 minutes for medium-rare doneness (140 degrees F) or 1 to 1-1/2 hours for medium doneness (155 degrees F). (For a gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking. Place lamb on rack in a roasting pan; place on grill rack. Cover; grill as above.)
Firmly pat crumb mixture on and around the top of the roast. Cover and grill 3 minutes more or until crumbs are golden. Cover with foil; let stand for 10 minutes before carving. (The meat's temperature will rise about 5 degrees F during standing.) Makes 4 servings.