Place all marinade ingredients in a blender and puree. Put pieces of skirt steak in a large ziplock bag and pour marinade all over. Seal the bag, removing as much air as possible and allow the steak to marinate for minimum 1 hour (and up to 8 hours) in the refrigerator. Marinate chicken for 30 min to 1.5 hours max.
Heat your grill to high heat and start grilling the steak immediately, while the fire is at its hottest. Grill on each side 3-5 minutes. Remove the steak from the grill and wrap in tin foil and allow to rest 10-15 mins. For chicken, grill over medium heat to retain its juiciness and not burn it.
While the steak or chicken is resting, toss the peppers and onion with some olive oil and a pinch of salt, and cook in a pan until they are soft and nicely browned.
Heat the tortillas on the grill or in a hot pan until warmed through.
Assemble fajitas with the vegetables, steak, sour cream, cheddar, salsa and avocado.
Alternative fillings: Grilled or Sautéed Mushrooms, Grilled or Sautéed Shrimp (same marinade)