Whisk the lime juice, oil, garlic, cumin, chili powder, and salt in a shallow dish. Add the steak and turn to coat. Rest for 30 minutes at room temperature or up to 4 hours refrigerated.
Grill and Slice:
Heat a grill or cast-iron skillet until very hot. Cook the steak for 4 to 6 minutes per side, depending on thickness, until browned and cooked to your preferred doneness.
Rest the steak for 5 minutes, then slice thinly across the grain.
Assemble:
Warm the tortillas. Fill with sliced beef and finish with onion, cilantro, salsa, and lime wedges.
Notes
Do not skip the resting time before slicing.
Warm tortillas in a dry skillet or wrapped in a towel.