In a large bowl, whisk together orange juice, lime juice, olive oil, minced garlic, diced onion, cumin, chili powder, salt, and pepper to create the marinade.

Place skirt steak in a large zip-top bag or shallow dish and pour marinade over the meat. Seal and refrigerate for at least 2 hours, or up to 24 hours for best results.

Remove steak from refrigerator 30 minutes before cooking to bring to room temperature. Preheat grill to high heat or heat a large cast-iron skillet over high heat until smoking.

Remove steak from marinade and pat dry with paper towels. Reserve some marinade if desired for serving.

Grill or sear the steak for 3-4 minutes per side for medium-rare, or until internal temperature reaches 125-130°F. Don't move the meat while cooking to achieve proper char.

Transfer steak to a cutting board and let rest for 5 minutes. Slice against the grain at a diagonal angle into thin strips.

Warm tortillas on the grill or in a dry skillet until slightly charred and pliable, about 15-30 seconds per side.

Serve immediately with warm tortillas, diced onion, fresh cilantro, and lime wedges. Let everyone build their own tacos.