Prepare the steak. Prepare your flank steak by seasoning it with 2 tablespoons of the fajitas seasoning mix. Drizzle 1 tablespoon olive oil over the steak and turn to coat.
Wrap the steak. Cut a large piece of aluminum foil. Place the steak in the center of the foil and fold up the sides so it holds in the juices, but don't close it shut. Transfer the steak to a sheet pan and set aside.
Prepare the veggies. Drizzle the peppers and onions with the remaining 1/2 tablespoon olive oil and toss with the remaining fajitas seasoning mix. Then, arrange the veggies on the sheet pan.
Cook. Transfer the sheet pan to the oven and cook for 12 minutes, or until the steak's internal temperature reaches 135˚F.
Broil. Turn your oven to broil. Stir the veggies and place the sheet pan under the broiler for 2 more minutes, or until the steak's internal temperature registers at 140-145°F.
Finish and serve. Remove the steak fajitas from the oven and let the steak rest for about 5 minutes before slicing. Top with cilantro or parsley and a squeeze of fresh lime juice. Serve with warm flour tortillas and toppings.