Pound both sides of steak with a tenderizer mallet. Poke several holes on each side of steak with a fork piercing several times.
Mix oil, onion soup, vinegar, pepper, garlic, mustard, and rosemary together in a bowl. Place half of mixture into a large plastic bag or Ziploc bag. Place steak on top of mixture. Pour remaining mixture over steak. Push any air out of the bag before closing. Distribute mixture well by kneafing with hands, Refrigerate For
Move rack to second position below broiler in oven. Preheat oven to broil At high heat. Place a cast iron grill, or regular cast iron pan in oven on rack. ( you may also use a broiler pan) heat pan until nice and hot. This should take about 10 minutes. Place steak on pan. Sear each side of steak for about 2-3 minutes on each side.vcontinue to broil each side for an additional 3-5 minutes on each side, or when thermometer reaches 135 degree's. This will be cooked medium.
Remove from oven to platter. Let the steak rest 10-15 minutes. Cut steak in to thin slices against the grain., and serve.