Season the skirt steak on both sides with salt and freshly ground black pepper.
Heat a skillet or grill pan over high heat until very hot, then swirl in the cooking oil.
Add the skirt steak and sear for 2–3 minutes per side for medium-rare if the steak is about 1/2" thick.
If the steak is thicker, reduce heat to medium, cover the pan, and cook an additional 2–3 minutes until desired doneness.
Remove the steak and let it rest for 5 minutes to retain juices.
Slice the skirt steak thinly across the grain into very thin ribbons.
Warm the tortillas if desired, then assemble each taco with steak, shredded lettuce, diced tomatoes, minced cilantro, and avocado slices.
Top with crumbled queso fresco or shredded cheese and serve with lime wedges.