Place the steak in a glass dish or resealable bag. Using a fork, poke hole all over the surface of the steak for the dressing to soak into.Pour the dressing over the steak and then add the onions. Turn the steak over to coat the other side making sure the entire steak is coated with the onions and the dressing.Marinate overnight in the refrigerator.Remove the steak from the marinade, wiping off the excess and discarding the onions.Grill for 5 minutes per side for medium rare. (this will depend on the thickness of your steak so always use a thermometer to check for the right temperature.)Remove the steak from the grill, cover with foil and let rest for 5 minutes.

Slice and serve.