Sprinkle steak with pepper on both sides, pressing it into meat. Broil or grill 3 inches from heat source: 3 minutes per side for rare, 4 minutes per side for medium-rare, and 5 minutes per side for medium.
Transfer the steak to a cutting board and let rest 3 minutes before cutting into slices 1/2-inch wide. Overlap the slices on a dinner plate and, if desired, serve with a generous dollop of mashed potatoes. Sprinkle all over with chopped parsley.