In a bowl, mix together the olive oil, minced garlic, lime juice, orange juice, cumin, chili powder, smoked paprika, oregano, salt, and pepper. Place the flank steak in a resealable plastic bag and pour the marinade over it. Seal the bag, ensuring the steak is well-coated, and refrigerate for at least 1 hour, or up to 8 hours for maximum flavor.
Preheat your grill to high heat. If using a carbon grill, allow it to reach a temperature of around 450-500°F (232-260°C).
Remove the steak from the marinade, letting excess marinade drip off. Place the steak on the grill and cook for about 4-5 minutes per side for medium-rare, or longer if you prefer it well done. Use a meat thermometer to check the internal temperature (130°F for medium-rare, 140°F for medium).
Once done, transfer the steak to a cutting board and let it rest for about 10 minutes. This will allow the juices to redistribute and ensure a tender bite.
Slice the steak thinly against the grain. Serve the slices in warm corn tortillas and top with diced onions, chopped cilantro, avocado slices, and lime wedges.
Notes
For best flavor, marinate the steak for up to 8 hours.