In a small bowl, whisk together the orange juice, olive oil, vinegar, chili powder, salt, onion powder, garlic powder, cumin, smoked paprika, and black pepper until well combined.
Add the flank steak and marinade into a large plastic storage bag or large baking dish. Massage the marinade into the steak until it's evenly coated, then seal or cover, and refrigerate for 2 hours, up to 8 hours.
Remove the steak from the marinade and shake off any excess liquid. Transfer it to a cutting board and lightly pat it dry with paper towels.
Lightly drizzle the steak with olive oil on both sides and season it with a small sprinkle of kosher salt and pepper on both sides.
To grill: Heat a grill on high heat. Add the flank steak and cook for 4 to 8 minutes per side depending on the thickness of the meat.
To cook on the stove: Heat a cast iron skillet or grill pan over medium-high heat. Add the flank steak and cook for 5-8 minutes on each side depending on the thickness of the meat.
When the steak’s doneness is to your liking (an internal temperature of 130°F for medium rare, 140°F for medium, 150°F for medium-well, and 160°F for well done), transfer it to a cutting board and let it rest for 5 minutes.
Slice it against the grain into thin strips and serve in warm corn or flour tortillas with a dollop of guacamole, diced onions, chopped cilantro, and your favorite fresh salsa or pico de gallo.
Notes
Vinegar: You could also use apple cider vinegar or white distilled vinegar.
Orange juice: If you don't have orange juice, you can use lime or pineapple juice instead.
Flank steak is best cooked to medium-rare. This temperature creates a juicy and tender flank steak that’s not chewy or dry. To do so, take the beef off the grill once it has reached 130°F and let it rest for 5 minutes. I like using an instant-read thermometer to measure the temperature.
Flank steak is best cooked on high heat for less time to ensure it's juicy and tender. If you don’t have a grill, a heavy skillet like a cast iron skillet works best to cook it over high heat on the stove. You could also use a grill pan to get those beautiful grill marks.