How to Make It
Stir together fajita seasoning, lime zest and juice, black pepper, 1/4 cup of the oil, and 1/2 teaspoon of the salt in a small bowl. Reserve 2 tablespoons fajita marinade in a separate bowl, and set aside. Place steak in a 1-gallon ziplock plastic bag. Pour in remaining fajita marinade, and seal bag. Marinate in refrigerator at least 1 hour or up to 4 hours, turning and massaging marinade into meat in sealed bag occasionally.
Heat 1 tablespoon of the oil in a large cast-iron skillet over high. Add onion-and-bell pepper mix; cook, stirring often, until slightly softened and caramelized, about 6 minutes. Sprinkle with remaining 1/2 teaspoon salt; stir in reserved 2 tablespoons fajita marinade. Cook, stirring constantly, 1 minute. Transfer onion mixture to a plate; cover with aluminum foil to keep warm. Wipe skillet clean.
Add remaining 1 tablespoon oil to skillet; heat over high. Remove steak from marinade, allowing any excess to drip off; discard marinade. Add steak to skillet; cook to desired degree of doneness, 5 to 6 minutes per side for medium-rare. Transfer to a cutting board. Tent with aluminum foil, and let rest 5 minutes.
Thinly slice steak across the grain, and arrange on a platter. Place onion mixture alongside steak. Serve with warm tortillas.