Thinly slice cabbage and toss with juice from 1 lime, a pinch of salt, and set aside; chop cilantro leaves and set aside.
Season fish with cumin, salt, and pepper, then sear skin-side up in olive oil over medium-high heat for 3 minutes; flip and cook 2 more minutes until internal temperature reaches 145°F.
Warm tortillas over a gas flame or in a dry skillet for 10 seconds per side, then place 3 on each plate.
Divide fish among tortillas and top each with cabbage slaw, cilantro, and a small dollop of lime-cilantro mayo (mix mayo with juice from remaining lime and 1 tablespoon cilantro).
Serve immediately with lime wedges.