Marinate Beef: In a gallon-size resealable bag or large bowl, add the garlic, OJ, lime juice, lemon juice, cilantro, and fajita spice. Season with ¾ teaspoon of salt and a heaping ¼ teaspoon black pepper. Remove and reserve ¼ cup of the marinade to use as sauce for serving sauce. Next, add the steak to the bag, and press to remove all excess air and seal the bag. From the outside of the bag, use your hands to massage the marinade into the beef, making sure the steak is well seasoned all over. Transfer the bag to the refrigerator and let marinate for at least 30 minutes or 24 hours.

Bring Steak to Room Temperature: 30 minutes before cooking, remove the bag of marinated beef from the refrigerator and place it on the counter. Let it come to room temperature.

Meanwhile, Heat Grill or Skillet: When you are ready to cook the steak, heat the grill, a grill pan, or a large cast iron skillet over high heat (450-degrees Fahrenheit)and oil the grates if applicable.

Prep Steak: Remove the steak from the marinade, discarding the marinade, and pat the steak completely dry. Season both sides of the steak generously with salt and pepper.

Cook Steak: Once the grill or pan is hot, add the marinated steak and sear for 3-5 minutes total, flipping steak over half-way through cooking, or until the steak reaches your desired degree of doneness. (Note: The exact cooking time will depend upon the size/thickness of your steak and your desired doneness. I cook mine for 3 minutes total and I like my steak mid-rare.)

Rest Steak: Remove steak from the skillet or grill and transfer to a clean work surface. Let rest for at least 3 minutes. Note: While steaks are resting, continue on with the recipe.)

Cook Veggies: Place the peppers and onions into a large bowl. Drizzle with a tablespoon of oil and season with fajita spice, if using. Toss to coat. Reduce heat to medium-high and add the veggies to a grill basket or place them directly onto the hot grill pan. Cook, flipping/stirring a couple times, for 4-6 minutes, or until slightly charred all over. Remove the veggies from heat.

Thinly Slice Skirt Steak: Use a sharp knife to thinly slice the marinated and grilled steak against the grain.

Assemble and Serve Fajitas: Divide the fajita steak and veggies among warm tortillas. Garnish with your favorite Mexican toppings and serve with reserved sauce on side. Enjoy immediately!