Combine all marinade ingredients in a ziplock bag, add steak and marinate for 1 hour at room temperature, or longer in the fridge.
Slice onions and peppers, set aside.
Preheat gas grill to high heat, then add cast iron skillet until hot.
Before cooking steak, wipe off excess marinade, and sprinkle steak with salt.
Add 1 Tbl. canola oil to hot cast iron skillet, then add steak searing for 2-3 minutes per side for medium rare. Remove to plate and cover with foil for at least 5 minutes before slicing.
While steak rests, add onions and peppers to cast iron skillet, searing until lightly browned on one side before stirring. Continue cooking for about 5 minutes.
Thinly slice steak at an angle across the grain.
Serve with fresh salsa, sauteed onions & peppers, shredded lettuce and cheese, sour cream, guacamole, and warm flour tortillas.