Combine all the marinade ingredients in a mixing bowl and mix thoroughly.
Place the steak and sliced onion in a ceramic or glass baking dish. Pour the marinade over the meat and rub it in well. Cover and refrigerate for 1–8 hours.
Clean the grill grates, grease them lightly with nonstick spray, and preheat the grill to 450°F.
Grill the steak for 4–5 minutes per side, until an instant-read thermometer registers 135°F for medium-rare.
Remove from the grill, tent loosely with foil, and rest for 10 minutes.