In a large, heavy-duty freezer ziptop bag, combine garlic paste, lime juice, cumin, and olive oil. Seal and shake to combine. Place skirt steak, bell peppers, onion, and cilantro sprigs into the bag, squeeze out the air, and turn to coat the meat, distributing vegetables evenly. Refrigerate at least 2 hours or overnight to allow marinade to penetrate.
Preheat the gas or charcoal grill to high heat.
While grill is heating, remove skirt steak from marinade to a platter, shaking off liquid and reserving marinade with the vegetables.
Preheat a heavy skillet to the smoking point. Drain vegetables and cilantro, and quickly stir-fry over high heat until slightly charred and softened, but not mushy. Set aside and keep warm.
Grill skirt steak over high heat quickly until medium rare, about 2 minutes per side. Slice meat against the grain into strips and serve immediately with warm tortillas, the seared pepper vegetable mix, guacamole, salsa, and sour cream.
To eat Mexican fajitas, diners traditionally place strips of meat with grilled vegetables, guacamole, salsa, and sour cream into the tortilla, and fold it up taco- or burrito-style.
Notes: Fajitas may also be made with chicken breast or pork tenderloin steaks. If you like your meat spicy, add 1 minced serrano chile to the marinade. If you do not have a gas or charcoal grill, you may use a cast iron or heavy grill pan on the stove-top. It is important that the pan be very hot before quickly searing the meat. An overhead exhaust fan is mandatory for indoor stove-top grilling.
Yield: 12 fajitas, serving 6