To make the marinade, in a blender mix the lime juice, Worcestershire sauce, garlic, cilantro, jalapeño, cumin, salt, black pepper, and cayenne. Place the steak in a non-reactive vessle or zipper bag, pour the marinade over the skirt steak, and let it marinate covered sealed for 2 to 8 hours in the refrigerator.
Before cooking, remove the steak from the refrgerator. Wipe off the marinade then let the steak come to room temperature, about 30 minutes.
To cook the steak, heat on high a large cast-iron skillet or grill pan (you may have to cut the steak in half to get it to fit) for about 4-5 minutes, or until a drop of watter sizzles and evaporates when you splash it into the pan. Also, turn on the broiler and place a rack 6 inches from the heating element. (You may also want to open a window close to the stove if you don't have a strong hood.)
When the skillet is hot, pour the oil into the skillet. Add the steak, then cook 3-4 minutes or until nicely browned on both sides, turning once. (If you have an insant-read thermometer, it should register at 130°.) Remove the steak from the skillet, and allow it to rest for 10 minutes.
While the steak rests, place the onions in the skillet, and then slide them under the broiler for 2 minutes or until charred.
After the meat has rested, thinly slice it against the grain. Serve with warm flour tortillas, charred green onions, and green salsa so people can make their own tacos.
Notes
I don’t have a grill or outdoor space, so I don’t know as much as I’d like about the subject, but to cook the steaks truly al carbon, I’ve been told to cook it 5-7 minutes, turning once.