1. In a mixing bowl, combine 3 tablespoons of lime juice, Worcestershire sauce, garlic, cumin, black pepper, oregano, oil, and 1/2 teaspoon salt.
2. Coat the skirt steak evenly on both sides with the marinade. Cover and refrigerate for about 1 hour. Avoid longer marinating to prevent the acid from changing the meat’s texture.
3. Preheat a gas grill to medium-high, or prepare charcoal coals until covered with gray ash.
4. Mash the avocados with the remaining 1 tablespoon lime juice and salt to taste. Prepare salsa and lime wedges.
5. Grill knob or sweet onions, if using, until tender and slightly charred. Warm tortillas and keep them wrapped to maintain softness.
6. Remove excess marinade from the steak. Place steak on the hottest part of the grill. Sear, without moving, for 3 to 4 minutes per side for medium doneness.
7. Allow steak to rest for a few minutes, then cut into 3-inch lengths. Slice thinly against the grain into 1/4-inch strips to ensure tenderness.
8. Serve the steak strips with mashed avocado, salsa, lime wedges, grilled onions (if using), and warm tortillas.