Remove fat from meat. Place in a bowl or plastic bag with vinegar and onion. Let marinate 20 minutes. Drain and place on a hot grill or under a preheated broiler. For rare, cook 5 minutes if steak is thin or 8 minutes if thicker. Cook longer if you like it medium or well done. Remove to a cutting board, cover with foil, and let rest 5 minutes. Slice on a diagonal against the grain.
This recipe provides 4 servings.