Marinate the Flank Steak: Whisk together the ingredients for the marinade in a medium sized bowl. Place the Flank Steak in a baggie and pour in the marinade, seal the baggie. Place in the fridge for 6 hours or overnight.
Meanwhile, you can slice the bell peppers and onions and set aside. Make the fajita seasoning by mixing all of the spices together in a bowl. Set aside.
Once the Flank Steak has marinated, remove it from the baggie and slice into thin strips (about ½ - ¾ inch wide strips). Be sure to slice against the grain of the meat. Set aside.
Heat a cast iron skillet over high heat. Add in 2 tablespoons of olive oil and let this heat for about 1 minute. Then add in the bell peppers and onions. Sauté until the veggies are tender (about 7 minutes). Add in 2 Tbsp. of the prepared fajita seasoning and toss to coat the veggies. Remove the veggies from the cast iron and set aside on a plate or dish.
Return the cast iron to the stove to cook the flank steak. Add the steak to the cast iron, season with 1 tablespoon of the fajita seasoning and cook, using tongs to toss the meat until the meat is done and cooked to your preference, about 4-5 minutes or until the outside surface of beef is no longer pink (Do not overcook).
Once the meat is cooked you can add the veggies back in and toss to combine. Serve on a warm tortilla with your favorite garnishes: avocado, guacamole, sour cream, salsa, cheese.