How to Make It
Preheat grill to medium-high (about 450°F). Stir together chili powder, cumin, pepper, and 1/4 teaspoon of the salt. Rub oil over steak, and coat with spice mixture. Let stand 10 minutes.
Meanwhile, to make the salsa, stir together corn kernels, avocados, chopped cilantro, lime zest, lime juice, and remaining 1/4 teaspoon salt.
Place steak on oiled grate, and grill, uncovered, to desired degree of doneness (2 to 3 minutes on each side for medium-rare). Let stand 5 minutes.
Place scallions on grate, and grill, uncovered and turning occasionally until charred and tender, about 2 minutes.
Cut steak across the grain into strips, and divide evenly among corn tortillas. Chop scallions, and sprinkle over steak. Top with salsa, and serve with lime wedges.