In a blender, blend cilantro, orange juice, oil, lime juice, jalapeño, garlic, cumin and sugar until smooth.
Pour marinade over steak; cover and refrigerate for 1 to 8 hours.
Heat barbecue to medium-high. Remove steak from marinade; grill, basting with marinade 2 or 3 times, for 8 to 10 min. for medium or until instant-read thermometer reads 160°F (71°C). Discard remaining marinade. Allow steak to rest 5 to 10 min. before carving against the grain into thin slices.
Wrap steak in tortilla with salsa, avocado, red pepper and onion. Squeeze lime wedge over top, and add a dollop of the sour cream (if using).
Double this marinade recipe and use it as a flavourful dressing for grilled vegetables. Serve veggies with tortillas, beans, rice and your favourite condiments for an easy vegetarian fajita option.
To warm tortillas, grill each side for 5 to 7 seconds.