Rub steak with seasoning mix. Brush onion and peppers with oil., Grill steak and vegetables, covered, on a greased rack over medium direct heat 4-6 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; and medium-well, 145°) and vegetables are tender. Remove from grill. Let steak stand, covered, 5 minutes before slicing., Cut vegetables and steak into strips; serve in tortillas. If desired, top with avocado and cilantro and serve with lime wedges.