To make the marinade, in a medium bowl whisk together all marinade ingredients. Add steak, cover with marinade, Refridgerate for 3 to 4 hours.
Cut an 18-inch square of heavy-duty aluminum foil. Place peppers and sliced onion in center of foil. Dot with butter. fold up foil, leaving a little space for expansion of steam, tightly seal top. Remove steak from marinade, Grill 10 to 15 minutes for rare, turning and brushing with reserved marinade once halfway through grilling time. Place foil packet of vegetables next to steak for last 10 minutes of grilling time.
Wrap tortillas in heavy-duty aluminum foil; and cook with steak for last 5 minutes of grilling. Turn tortilla packet once halfway through grilling. Serve with peppers and onions, salsa, and guacamole.Servings: 6