You will need:
500 g Firm, white fish that won't break apart easily, preferably line fish of some sort. Tuna is excellent, as is barracuda or other line fish.
2 Large onions peeled and sliced.
2 fresh green chillies sliced in half lengthways.
A handful of fresh garlic cloves, peeled and roughly chopped.
3 cm of fresh ginger, peeled and roughly chopped.
1-2 cups good quality white wine or apple cider vinegar
2 Tsp Black Mustard Seeds (kalwanji)
Fresh Coriander leaves.
Proceed as follows:
Put into the sliced onions into a bowl bowl and cover with the vinegar for an hour or so.
Pat the fish dry with paper towels. Cut into large porthions, take out any bones and season with salt & pepper.
Heat about 5 mm of oil over a low heat in a large, heavy bottomed frying pan. Fry the fish for a few minutes on both sides, just to crisp the outside. Don't overcook or it will dry out. Remove the fish from the oil, and allow it to drain on paper towels. Wipe the pan clean
Put the peeled and roughly chopped garlic and ginger into a blender with the mustard seeds and grind to a pulp or use a mortar and pestle (better). Crush as many of the seeds as possible.
Pour 5mm of fresh oil into the frying pan and warm through slowly and gently. Important - don't heat the oil too much, you are not going to be frying this, just coaxing out the the flavour. Add the drained onion, the garlic/ginger/mustard seed mix, the sliced chillies, and the turmeric (say about 20 ml). Stir well, and add several tablespoons of the vinegar that the onions were soaking in. The aroma will be fantastic. Cook and stir for about 5 minutes, just until the onion is soft. Add 5 cc of sugar stir well. This is your marinade.
Remove the marinade from the heat. Add the pieces of fish and spoon the marinade over, try not to break the fish up. Check the seasoning and add salt, pepper, and possibly some more of the vinegar.
Cover. cool and refrigerate for 48 hrs, if you can wait that long. It will be worth it.
This is served at room temperature, depending of course where your room is, take it out of the fridge a good while before serving. Just before serving stir in some chopped coriander to taste. If you don't like coriander, leave it out. Put some good crunchy french bread and lots of butter on the table to mop up the juices. Beer or chardonnay to drink.