Place the flank steak on a cutting board and cover it with plastic wrap and pound the steak with a meat tenderizer until the whole piece of meat is about ¾-inch thick. Once tender, season the steak with salt on both sides. Massage the salt into the meat and set the steak aside.
Next, thinly slice the bell peppers, purple onion, and garlic. We suggest using a kitchen mandolin to evenly slice the vegetables. If you don’t have a mandolin, use a very sharp knife to slice the vegetables. Set aside.
Slice the flank steak into thin strips, using a sharp knife to slice the steak against the grain of the steak.
Heat a large skillet with sides over medium/high heat and add the avocado oil to the pan. When the avocado oil is fragrant and hot, add the sliced steak to the pan. Sprinkle 1.5 teaspoons of fajita seasoning over the steak. Cook the steak for 2-3 minutes, tossing the steak occasionally.
Remove the steak from the pan with a slotted spoon (leaving the steak juices and drippings in the pan) when it still has specks of pink on some pieces. The steak will continue to cook later. Set the steak aside.
Add the onion and garlic to the skillet and sauté in the beef drippings and avocado oil. Sauté the onion and garlic for 2 minutes and then add all of the sliced peppers to the skillet and toss with the onion and garlic. Season the vegetables with the remaining 1.5 teaspoons of fajita seasonings. Cover the skillet and sauté the peppers for 5-6 minutes, removing the cover to toss the peppers periodically.
Once the peppers begin to soften, add the steak to the skillet and cook for an additional minute or until the steak reaches desired doneness.
Remove the skillet from the heat.
Serve the fajita steak and vegetables in tortillas with optional toppings.