In a mixing bowl, whisk together olive oil, lime juice, garlic, cumin, chili powder, salt, black pepper, and cilantro.

Place the flank steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, or up to 8 hours.

Preheat the grill to medium-high heat.

Remove the steak from the marinade and discard the marinade. Grill the steak for about 5 minutes on each side, or until it reaches your desired level of doneness.

Let the steak rest for 5 minutes before slicing it against the grain.

Warm the corn tortillas on the grill for about 30 seconds on each side.

Serve the sliced steak on the tortillas topped with diced onion, chopped cilantro, and lime wedges.