Preheat the oven to 220C/200C Fan/Gas 7 for the fish, and begin by preparing the sides for your tacos. Put the red onion slices into a bowl and cover with the lime juice. Stir with a fork to mix the onion well in the juice. Actually, if you can get this done ahead of time (up to 1 day), so much the better, but even 20 minutes is fine.
Make the hot sauce by mixing together the mayonnaise and the gochujang paste (or other hot sauce) in a small bowl. Set aside.
For the corn relish, put the canned sweetcorn into a bowl. Add the chilli to the sweetcorn, along with the chopped coriander and some salt to taste. Give it all a good stir, and set aside.
Cut the fish fillets in half lengthways, so you have long fingers, and arrange in a shallow roasting tin. Mix together the cumin, paprika and salt, and sprinkle over the fish fillets. Mix the garlic and the oil in a small bowl. Drizzle the fish with the garlicky oil, and roast in the oven for 8–10 minutes, depending on the thickness of your fillets. Check to see if the fish is cooked through (though only just) before taking out of the oven.
Once you’ve taken the fish out (though don’t let it get cold) turn off the oven and warm the tortillas in the fading heat. Meanwhile, peel, stone and slice the avocados, then spritz with the juice of the lime, ready to serve with the other accompaniments.
Arrange the fish on a plate (lined with salad leaves, if so desired), sprinkle with some chopped fresh coriander, and take to the table along with the warmed tortillas. If you don’t want to add any salad to the fish directly, you may want to put a bowl of shredded iceberg or some other crisp leaves nearby.
Put the bowls of soused, but drained, red onions, the corn relish, hot sauce and a plate of sliced avocados on the table so that everyone can load their tortillas. A plate of lime wedges and a pile of napkins are both good ideas, too.