Preheat the oven to 220°C and soak sliced red onions in lime juice.
Whisk mayonnaise and gochujang paste together in a small bowl.
Combine sweetcorn, diced chilli, coriander, and salt to create the corn relish.
Slice fish into fingers and place in a roasting tin.
Season fish with cumin, paprika, and salt, then drizzle with garlicky olive oil.
Roast the fish for 8 to 10 minutes until just cooked through.
Warm tortillas in the oven and slice avocados, drizzling them with lime juice.
Serve fish with tortillas, pickled onions, corn relish, hot sauce, and avocado.