Sliver 1/4 of the onion and set aside. Chop the rest of the onion. Mis 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.
Sprinkle both sides of the steak with teh salt and pepper; prick well with a fork. In a large zip-top bag, comvine the steak with the remaining onion mixture. Marinate for 1 hour to overnight.
Heat the grill or broiler, positioning the oven brioler rack so that the meat on the rack in the pan is 4" fromt he heat source. Removethe meat from the marinade, and place on grill over direct heat or on an oven rack set in the broiler pan. Discard marinade. Grill or broil for 4-5 min per side for med rare. Let stand 5 min before slice.
Place meat on a platter and pour the reserved onion mixture over the steak.