Place the sliced flank steak in a large, zip-top bag. Set aside 2 teaspoons of the fajita seasoning.
In a small bowl, whisk together the remaining fajita seasoning, 2 tablespoons of olive oil, garlic, lime juice, and salt and pepper, to taste. Add to sliced steak in bag and seal. Refrigerate for at least 2 hours, or up to 24 hours.
Meanwhile, place sliced peppers and onions in a medium bowl. Toss with remaining tablespoon of oil and remaining fajita seasoning. Season with salt and pepper, to taste.
Preheat grill to medium-high heat. Tear off 6-8 large sheets of aluminum foil. Spray each sheet with non-stick cooking spray. Dived flank steak, peppers and onions evenly between foil sheets. Seal foil tightly around fajita mixture.
Grill for 6-8 minutes per side, or until peppers are tender and steak is cooked to desired doneness.
Serve in flour tortillas with your choice of toppings.