Make the marinade. In a mixing bowl, combine all the ingredients except for the steak and onions and mix well.
Marinate the steak. Place the steak in a ceramic or glass baking dish with the onion and pour the marinade over the steak. Rub the marinade into the steak really well, cover, and refrigerate for at least 1 hour, not longer than 8 hours.
Prep your grill. Before using your grill, make sure to clean the grates of any food and debris. Lightly grease the grates with nonstick spray. Set the temperature to medium-high. Close the lid and wait until the temperature reaches 450 F.
Grill the steak. Place the marinated steak on the prepared grill, disposing any excess marinade. Grill the steak for 4-5 minutes per side, or until an instant-read thermometer inserted in the center of the steak registers an internal temperature of 135 degrees F.
Rest and slice. Remove the steak from the grill and lightly tent with foil. Let the steak rest for 10 minutes, then slice against the grain. Serve the carne asada warm.
