Preheat the grill to 400 degrees F (205 degrees C).
Sprinkle the flank steak liberally with the seasoning and let rest at room temperature for 30 minutes.
Grill the flank steak until medium-rare, 135 degrees F (55 degrees C) using an instant-read thermometer, about 4 minutes per side. Remove the steak to a cutting board and let rest for 10 minutes.
Meanwhile, toast the tortillas on the grill, for about 30 seconds per side.
Slice the flank steak thinly against the grain. Serve it on top of the tortillas, with the onions, cilantro, cheese, and lime wedges.