Transfer skirt steak to a cutting board. Cut steak in half, or into 10- to 12-inch long sections so that each piece fits into a large frying pan. Rub each piece of steak all over with fajita seasoning and set aside.
In a large skillet heat 2 tablespoons of olive oil over medium-high heat until shimmering. Add steak pieces, or cook in batches if all pieces will not fit together in pan to prevent overcrowding. Cook each side 2 to 3 minutes.
Transfer steak to a clean cutting board and set aside to rest.
In a medium bowl toss together black beans, corn, red onion, cilantro, tomato, garlic, ½ teaspoon salt, and half of the lime juice until well combined. Set aside.
In a small bowl stir together sour cream, chipotle in adobo, 1 tablespoon of olive oil, red wine vinegar, remaining lime juice, garlic powder, and ½ teaspoon salt until well combined.
Slice skirt steak against the grain into ¼-inch thick slices.
In a large bowl, add spring mix. Top with black bean and corn salsa and dressing and toss to combine.
Add steak and avocado to salad. Top with more fresh cilantro, and also with crispy tortilla strips and queso fresco, if using. Serve cold or at room temperature.
