For this method, place the thawed steak in a ziploc bag with all marinade ingredients. "Massage" the bag to coat the steak well. Allow to marinade 4-12 hours (the marinade time is longer on this option because the steak hasn't been sliced).
When ready to cook, heat the grill to high heat. Slice the peppers and onions into strips.
Remove the steak from the bag, allowing excess marinade to drip off and back into the bag. Place the steak on the grill, then put the peppers and onions in the bag with the marinade.
As the steak grills, heat a cast iron pan to medium high heat and add the onions, peppers, and marinade. Stir occasionally as the onions and peppers cook.
When the steak reaches an internal temperature of 130 degrees F for rare, 135 for medium rare, or 138 for well done, remove from heat. Place on a plate and cover. Allow to rest for 5-10 minutes as you continue to cook onions and peppers until tender.
Remove onions and peppers from heat.
Slice the steak against the grain, then toss the sliced steak with the onions and peppers until combined.
Serve on taco shells with guacamole and cilantro for garnish.