Combine Cilantro Lime Sauce: add ingredients to a small food processor. Blend until smooth. Taste and add additional cilantro, lime or salt - as needed! Cover and place in the refrigerator until serving.
Prepare Steak: Pat 1 ½ pounds flank steak dry with clean paper towels. Season with salt and pepper - pressing the salt and pepper into the meat. Place steak in a large reusable silicone bag or glass baking dish and set aside.
Marinate Steak: Add ⅓ cup fresh lime juice, ¼ cup extra virgin olive oil, zest of 2 limes, 1 tablespoon garlic, 1 tablespoon soy sauce, ½ teaspoon chili powder, ½ teaspoon ground cumin, ½ teaspoon kosher salt and 1-2 tablespoons cilantro (if using) into a small mixing bowl. Whisk until well combined. Pour the marinade over the steak, ensuring it is fully coated. Seal and place in the refrigerator for 30 minutes or more.
Cook Steak: Bring a large cast iron skillet to medium-high heat. Add a oil, swirl to coat the pan. Remove flank steak from marinade letting excess marinade drip off the steak; discard marinade. Place steak in the pan and sear 3-4 minutes per side. Take internal temperature and then cook 1-2 minutes per side until the internal temperature towards the thickest part of the steak reaches 130-132℉ for a medium-rare finish (use an internal meat thermometer for accuracy) - during the cooking process turn down the heat, as-needed, if your skillet really gets hot and/or add a little extra oil if the pan is dry - to avoid burning. Remove from skillet and let the meat rest on a cutting board for 10 minutes while you cook the veggies.
Cook Veggies: Keeping the pan at medium-high heat add 2-3 tablespoons chicken broth to deglaze the pan - use a spatula to scrape up the bits from the bottom of the pan. Add 1 tablespoon garlic and move around pan until it becomes fragrant, approx. 1 minute. Add onion, bell peppers and 2-3 tablespoons fajita seasoning. Stir to combine. Add additional broth or olive oil to pan if you feel it is getting a bit dry.
Finish Veggies: Sauté for approx. 10-15 minutes or until the peppers have reached your desired doneness (cook shorter for a more firm, but tender pepper - longer for a softer pepper). Taste and add any additional kosher salt and ground black pepper, to taste.
Serve: Slice the steak against the grain into thin slices and serve on warm tortillas with the fajita veggies, cilantro lime sauce and any other toppings you desire!
Notes
Steak Options: if you don't want to use flank steak - sirloin, skirt or hanger steak could also be used - just adjust cook time and temp as-needed.
Storage: place in an airtight container in the fridge for 3-4 days. We recommend storing the meat, veggies, sauce, salsa, etc. in separate containers to reheat easier the next day and to prevent sogginess.