Add the steak to a large Ziplock bag.
1½ pounds flank steak
In a medium bowl, whisk the lime juice, orange juice, olive oil, cilantro, garlic, cumin, salt, and pepper together, then pour the marinade over the steak.
¼ cup fresh lime juice, ¼ cup freshly squeezed orange juice, ¼ cup olive oil, ¼ cup chopped fresh cilantro, 4 cloves garlic, 1 teaspoon ground cumin, ½ teaspoon kosher salt, ¼ teaspoon ground black pepper
Toss and massage to coat the steak well. Marinate in the refrigerator for 1 hour and up to overnight.
Heat your grill or indoor grill pan over high heat. Grill the steak, discarding excess marinade, for 7-10 minutes per side. Set aside on a large plate or sheet pan and tent loosely with foil. Let the steak rest for 10 minutes.
While the steak rests, grill the tortillas until they are warmed through.
8 corn tortillas
Once rested, slice the steak against the grain. Serve in warm tortillas with red onion, avocado, and cilantro.
¼ cup diced red onion, 1 avocado, ¼ cup chopped fresh cilantro