Directions

1. Sliver one-quarter of the onion and set aside. Chop the rest of the onion. Mix it in a bowl with the vinegar, capers, oregano, and garlic. Combine 1/4 cup of this mixture with the slivered onions and set aside.
2. Sprinkle both sides of the steak with the salt and pepper; prick well with a fork. In a large zip-top food-storage bag, combine the steak with the remaining onion mixture. Marinate for 1 hour or overnight.
3. Heat the grill or the broiler, positioning the oven broiler rack so that the meat on the rack in the pan is 4 inches from the heat source. Remove the meat from the marinade, and place on the grill over direct heat or on an oven rack set in the broiler pan. Discard the marinade. Grill or broil for 4 to 5 minutes per side for medium-rare. Let stand for 5 minutes before slicing.
4. Place the meat on a platter and pour the reserved onion mixture over the steak.