Make the onions as far in advance as you can: at least two hours, and up to 24. Cut the red onion half – or use a whole onion, because you will find yourself adding them to much else besides – into fine half-moons. Put these in a jar with a lid, or simply in a bowl that you can cover.
Pour over the vinegar (or lime juice), pressing down on the onions so they are just immersed. Put the lid on the jar (or cover the bowl), and leave to steep.
When you’re ready to eat the bhorta, heat the oven to 220C (200C fan)/425F/gas 7. Peel and slice the two onions into fine half-moons, deseed the chillies (or not, if you prefer), slice finely, then peel the garlic. Peel the ginger then grate it finely to give one tablespoonful.
When the oven’s hot and your ingredients are assembled, put the fish fingers on a baking sheet and cook for about 20–25 minutes, which may be slightly longer than the packet directs, but will ensure the breadcrumb coating is really crisp.
Meanwhile, warm the oil in a large frying pan (I use a wok-shaped stir-fry pan), and cook the onions over a medium-low heat for 20 minutes, stirring regularly, by which time they will be pale gold and soft.
Add the sliced chillies, stirring all the while, for three minutes, then stir in the grated ginger, mince or grate in the garlic, and cook, stirring, for another two minutes.
Stir in the mustard and salt, add the spinach, let it wilt in the pan for two to three minutes, stirring regularly, then squeeze in the juice of the lime.
Take the pan off the heat while you get the fish fingers. Break them up a bit with a spatula, then add them to the wok or frying pan. Toss everything together, breaking them up further and mashing them into the pan, then sprinkle over the coriander.
Serve topped with the pink-pickled onions, adding extra chopped coriander, if wished.