Stir together the first 8 ingredients in a medium bowl to make the marinade. Combine the steak and 3/4 cup of the marinade in a large zip-top bag. Place the peppers and onion in another large zip-top bag, and pour in the remaining marinade. Seal the bags and marinate in the refrigerator for 2 to 4 hours, turning occasionally.
Coat a grill rack or grill pan with vegetable cooking spray. Remove the steak and vegetables from the bags and discard the marinade. Grill the steak 8 minutes per side and the vegetables 8 to 10 minutes, turning them every 2 to 3 minutes. Remove the steak and vegetables from the grill. Keep the vegetables warm and let the steak sit for 10 minutes before slicing.
Wrap the tortillas in foil and place on the grill or a grill pan to warm for a few minutes. Cut the steak on an angle, against the grain, into thin slices. Fill the tortillas with peppers, onion, and steak. Top with salsa, diced avocado, or sour cream (if desired).
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