Heat broiler. Rub both sides of lamb chops with rosemary, salt and pepper. Place lamb on broiler pan; broil until chops are browned, turning once, about 4 minutes per side for medium-rare.
Meanwhile, in a small bowl, whisk together lemon juice, oil, and mustard. Stir in red bell pepper, fresh mint, and scallion. Serve lamb chops warm, with sauce spooned over the top.