Make the marinade. In a medium mixing bowl, whisk together the garlic, lime juice, orange juice, jalapeÃ±o (if using), cilantro, oil, chili powder, cumin and oregano until combined.
Marinate the steak. Place the flank steak in a shallow baking dish, pour the marinade evenly over the steak, and toss the steak until it is evenly coated in the marinade. (Alternately, you can combine the steak and marinade in a large Ziplock or Stasher bag and toss to coat.) Cover and refrigerate for 2 to 4 hours to marinate.
Bring the steak back to room temperature. Remove the dish from the refrigerator, lift the steak out of the marinade and transfer it to a clean plate. Season each side with a few generous pinches of salt and pepper, then let the steak rest for 30 minutes or until it reaches room temperature.
Cook the steak. Heat an outdoor grill or indoor grill pan (or cast-iron skillet or griddle) to high heat. Cook the steak for 5-7 minutes per side â€” resisting the urge to move the steak as it cooks so that it can sear properly â€” until it reaches your desired level of doneness. (See temperature chart below.)
Rest the steak. Transfer the steak to a clean plate and let it rest for 10 minutes, which will help to seal in the juices.
Slice/cut and serve. Then slice the steak against the grain as thickly or thinly as you prefer. (Or you can dice the steak into small pieces.) Then serve and enjoy!