Mix the pre-chopped parsley and garlic in a small bowl. Add the orange zest and mix thoroughly. Set aside.
Spread salt and pepper evenly over the steak. Generously oil the grill grate. Place the red onion wedges and steak on the grate. Grill the beef for 5 minutes on each side.
Remove the steak from the grill, let it rest for 3-5 minutes to seal in the juices and prevent the steak from dripping when cut, then cut into 3cm long slices.
In a small saucepan, combine vinegar and shallots. Bring to a boil. Cook for about 7 minutes, then pour over steak slices.