In a small bowl, create dry rub for flank steak by whisking together all of the spices. Rub spice mixture all over both sides of the steak and let sit for at least 30 minutes.
Meanwhile, toss onion and pepper slices with olive oil, salt, and pepper. Take two large sheets of tinfoil and create a large, overlapping square. Place peppers and onions in the middle and create a sealed pouch to throw on the grill or into the oven.
Pre-heat grill to medium high, or set oven to broil.
Once heated, add flank steak and veggie packet to grill or oven. For medium-rare, cook steak about 4 minutes per side, turning once.
Cook veggies for 10 minutes, flipping once.
Place corn tortillas in tin foil and seal before throwing on the grill during last minute to warm. You could also warm them on the stove.
Serve tacos with sliced flank steak, peppers, onions and toppings of choice. (I made a quick pico de gallo and guacamole for mine.)