Slice the red onion into thin strips and submerge them in cold water for at least 15 minutes.
Slice the cabbage and shred carrots into a large bowl. Zest and juice both the limes.
Mince the garlic with 1/4 teaspoon of salt then use the edge of your knife to scrape it into a paste.
Combine the garlic paste with 4 tablespoons of lime juice with 2 tablespoons of olive oil, and 1/4 teaspoon cumin.
Drain the onions and add to the cabbage and carrots.
Toss the lime dressing with the cabbage mix, then let sit for 5 minutes. Taste after that then add a pinch of salt or more lime juice if needed.
Season the rockfish with chili powder, salt, lime zest, remaining cumin, and garlic powder.
Crisp the corn tortillas in 3 tablespoons of oil and set to drain over paper towels.
Heat a skillet over medium high heat and add the remaining tablespoon of oil.
Sear the rockfish filets for 2 minutes per side, then remove to a plate.
Assemble tacos with fish, then slaw, and cilantro if desired. Squeeze over more lime juice and enjoy.
Notes
*I highly recommend an extra lime sliced into wedges for drizzling over the finished tacos*Don't use a grater for the carrots. If shredded too thin or finely they become limp in the slaw.*To make the garlic paste, drag your knife firmly over the minced garlic after adding the salt. Scrape it together and repeat until it's crushed.*Instead of crisp corn tortillas, you can use warmed or grilled flour or corn tortillas instead.