1To make the marinade, combine all of the ingredients in a bowl and whisk together until combined. Reserve 1/4 cup of the marinade in an airtight container in the refrigerator.
2Put the tenderloin in a large zipper-lock plastic bag and add the remaining marinade. Seal and shake the bag to make sure the meat is well coated. Place in a baking dish and refrigerate for 8 hours or up to overnight, turning the bag a few times.
3When ready to cook, in a bowl, toss the peppers and onion with the reserved 1/4 cup marinade. Remove the tenderloin from the marinade, letting the excess marinade drip off, and transfer to a large plate. Let come to room temperature while you prepare the grill.
4Prepare a charcoal grill for medium-high heat, banking the coals on one side of the grill. Clean and oil the grill grate.
5Put a large cast iron frying pan on the cooler side of the grill and add the onions and peppers. Place the tenderloin over the hotter side of the grill. Cover the grill and cook, turning the tenderloin a few times, until it is nicely seared and cooked to medium (about 145°F). Transfer the tenderloin to a cutting board, tent with foil, and rest while you finish the vegetables.
6Move the cast iron frying pan to the hotter side of the grill and cook the vegetables, stirring occasionally, until tender. Set aside.
7Thinly slice the tenderloin against the grain, then transfer it to the cast iron frying pan with the vegetables. Serve right away with warm tortillas, salsa, sour cream, sliced avocado, and crumbled queso fresco.