Mix all marinade ingredients in a zip-top bag. Reserve two tablespoons of the mixture for the fajita vegetables. Prep the steak by removing any membranes and patting dry with paper toweling. Add the meat to the marinade. Seal the bag, removing as much air as possible. Massage the marinade into the meat. Marinate, refrigerated, for at least 4 hours (can be marinated overnight). Turn the meat occasionally if you can.
Set a cast-iron skillet over medium-high heat and add 2 tbsp. of vegetable oil (see Recipe Note #2). Remove the meat from the marinade, removing as much marinade as possible (I use a silicone spatula to scrape off any solid bits). When the oil begins to shimmer, add the meat to the pan. Brown well on each side, about 2-3 minutes per side for medium rare. Remove the steak to a cutting board and let rest while making the fajita vegetables.
Add 1 tbsp of oil to the skillet (do not wipe the skillet out). Add the fajita vegetables and sauté, stirring frequently, until soft, about 8 minutes. Toss with the reserved marinade and remove from the heat.
Cut the steak into manageable sections WITH the grain. Thinly slice each section ACROSS the grain. Cut slices into bite-size pieces.
Arrange the steak and vegetables on a platter and set out buffet style with warm tortillas and condiments.