In a large mixing bowl, combine the steak with salt, pepper, garlic powder, cumin, chili powder, oregano, and steak seasoning, if using. Toss until the meat is well coated. Let the seasoned steak sit at room temperature for 10 minutes while preparing the pan.
Heat a 12-inch/30 cm cast iron skillet or stainless steel sauté pan over medium-high heat. Add the oil. When the oil is shimmering but not smoking, add the steak in a single layer. Cook without moving for 2 to 3 minutes, then stir or flip to sear the other sides. The meat should be browned on the outside but still tender inside. Do not overcook. Transfer to a plate and set aside.
Warm the tortillas in a dry skillet over medium heat, directly over a gas flame, or wrapped in foil in a 300°F/150°C oven until soft and pliable.
In a small bowl, mash the avocados and stir in the lime juice, salt, onion, tomato, jalapeño, and cilantro. Mix well and adjust seasoning to taste.
Roast the tomatillos, garlic cloves, and jalapeño in a dry skillet or under a broiler until blistered and softened. Blend with cilantro, water, lime juice, and salt to taste.
To assemble, spread a spoonful of guacamole across the bottom of each tortilla. Top with a handful of shredded lettuce and a portion of seared steak. Finish with a generous drizzle of salsa verde. Serve immediately.