Make the marinade by combining the garlic, vegetable oil, mayonnaise, lime juice, cumin, adobo, and salt in a large glass or Pyrex bowl.
Cut the steak into 5-6 inch sections, or whatever will fit in your skillet. Place steak in marinade and make sure the marinade covers the meat as thoroughly as possible. Marinade for a minimum of 2 hours; I usually let it sit in the marinade, in the refrigerator, overnight.
Heat your cast-iron skillet over medium-high heat. When it’s hot, place the skirt steak pieces on the skillet and cook until medium-rare, about 10 minutes on each side. The meat should start to develop a slight “crust” on the outside.
Remove meat, slice against the grain into strips, and (optionally) return to pan to finish cooking.
At this point, add the green onions (full length or cut in half; not chopped) as well and let them cook in the grease from the meat. Add more vegetable oil if needed.
Serve on warm flour tortillas with Mexican rice, salsa, cheese, sour cream, lettuce, diced tomatoes, etc.