Turn on grill to medium-high heat and allow to heat up while you prepare the meat and vegetables.
Liberally rub the flank steak with spice rub on both sides, gently patting to help the spices stick. Sprinkle the sliced peppers and onion with olive oil, then with spices.
Place the flank steak on the grill and grill for 3-4 minutes per side. Test the temperature with a thermometer and pull the steak when it reaches 135 °F (for medium rare). The vegetables can go on at the same time on a cooking rack, or cooked on the stove in a skillet if you don't have a cooking rack. Flip the steak once and keep an eye on the vegetables so they don't get too charred.
Remove the steak to a cutting board and allow it to rest for a few minutes before slicing against the grain in 1/4 inch slices.
The tortillas can be heated in a skillet before serving. Top with your favorite pico de gallo, grilled vegetables, cheeses....it's your taco!! Enjoy!