Marinate the meat. Lay the skirt steak into a stainless or glass baking dish. In a bowl, stir together 3 tablespoons of the lime juice, the Worcestershire, garlic, cumin, black pepper, oregano, oil and ½ teaspoon salt. Scrape onto the meat, spreading it to coat both sides. Cover and refrigerate for an hour or so—any longer and the acid in the marinade will begin to leach the color from the meat.
Prepare the grill. About a half hour before cooking, heat a gas grill to medium-high or prepare a charcoal fire, letting the coals burn until they are covered with a gray ash and still quite hot.
The accompaniments. In a small bowl, mash the avocado with the remaining 1 tablespoon of lime juice. Season highly with salt (you’ll be using this as a condiment), usually about 1 teaspoon. Scoop into a serving bowl. Do the same with your chosen salsa and the lime wedges. Grill the knob onions, if you’ve chosen to serve them. Heat the tortillas; keep them well wrapped or covered to stay warm.
Grill the meat. Smear most of the marinade off the meat, then lay it on the hottest part of the grill. Let sear without moving until there are dark grill marks underneath, then flip and sear the other side. You’ll probably want to move the meat to the coolest part of the grill to coast toward your perfect doneness: For me, that’s medium (even though I like a ribeye medium-rare) because that’s when skirt steak is most tender but still juicy. Most pieces of outer skirt steak will cook to medium in 3 to 4 minutes per side.
Serve. Cut the long pieces of skirt steak into 3-inch pieces, then turn each piece and cut each piece across the grain into ¼-inch strips. (Cutting skirt steak with the grain yields unchewably tough pieces.) Sprinkle the strips of skirt steak with salt (a good place for a nice finishing salt) and scoop into a warm serving dish. Set on the table with the mashed avocado, salsa, limes and tortillas for everyone to make tacos. Serve the knob onions on the side, if you have them.
For the professional: Because this marinade has enough acid to leach color from the meat in a couple of hours, it’s not ideal to use for a service that goes on for 5 or more hours. A better choice is to smear the meat with the Yucatecan Spice Marinade (page 000), which is much less acidic, and use a spray bottle of lime while the meat is on the grill.