Place the flank or skirt steak into a large resealable plastic bag.
In a medium bowl, whisk together the lime juice, orange juice, olive oil, chopped cilantro, minced garlic, ground cumin, kosher salt, and black pepper to make the marinade.
Pour the marinade into the bag with the steak, press out excess air, then massage the bag so the steak is well coated.
Refrigerate and marinate the steak for at least 1 hour or up to overnight for more flavor.
Preheat a grill or indoor grill pan over high heat and discard any excess marinade from the steak.
Grill the steak 7–10 minutes per side, or until it reaches your desired doneness.
Transfer the steak to a plate or sheet pan, tent loosely with foil, and let it rest for 10 minutes.
While the steak rests, warm the corn tortillas on the grill or grill pan until heated through and slightly charred.
Slice the rested steak thinly against the grain.
Assemble the tacos by placing sliced steak in warm tortillas and topping with diced red onion, sliced avocado, and chopped cilantro.