Remove the steak from the fridge about 30 minutes before cooking to allow it to reach room temperature. Pat dry with paper towels to remove excess marinade. Generously season both sides of the steak with salt and pepper.
Preheat your cast iron skillet over medium-high heat. Add a bit of oil and when it starts to smoke, add your steak. Cook for about 3-5 minutes on each side for medium-rare, or until your desired doneness.
Once done, remove the steak from the skillet and let it rest for about 10 minutes. Slice the rested steak against the grain into thin strips. It is VERY IMPORTANT to slice against the grain, otherwise your steak will be chewy and rubbery and tough.
In the same skillet, add more oil if needed, then add bell peppers and onions, season with salt and pepper and squeeze half a lime. Sauté over medium heat until they are soft and slightly caramelized, about 10 minutes.
To serve, place some steak strips onto each tortilla, top with the sautéed peppers and onions. Add a dollop of sour cream, pico de gallo, avocado or guacamole, a sprinkle of fresh cilantro, and a squeeze of lime.