Cut the fish into bite size chunks.
Put the fish in a bowl. Add 1/2 tsp turmeric, 1/2 tsp salt, 3 grinds pepper. Mix this all together till the fish chuks are coated in the spices.
Put the 500 ml of cooking oil in the wok and turn the neat up high on the stove. Let it heat up for 3 or 4 minutes. Test the oil is ready by putting a piece of fish into it. If it sizzles and bubbles, it's ready. If not let it heat up a couple of minutes more and see if a piece of fish sizzles. It should by now. If not you may need to urn up the heat
Deep fry the fish in small batches for 1 minute per batch.
While the first batch of fish is frying line a baking tray or a plate with some kitchen/absorbent paper.
As each batch of fish is fried, lift it out with a slotted spoon and put it on the kitchen/absorbent paper which will absorb some of the oil.
Your fried fish should look something like this - lightly golden in colour.
Get some more sheets of kitchen/absorbent paper and cover the fish with it. This is to soak up some of the oil from the top side of the fried fish.
Chop 50 gm of onion, 2 cloves of garlic and 2 thin slices of ginger into small pieces.
Strip the leaflets off the curry leaf sprig.
Chop the lemongrass stalk into a few pieces.
Put 3 or 4 tablespoons of the oil you have fried the fish in into a sauce on a high heat on the stove.
Add to the saucepan the 50 gms chopped onion, the 2 cloves of garlic chopped, the 2 thin slices of ginger chopped and the leaflets of curry leaves. Let this cook for 1 or 2 minutes.
Add to the saucepan the roasted spices, cinnamon stick, lemongrass, fenugreek seeds, and let this all cook for 1 or 3 minutes.
Chop the chillies very small and add them to the saucepan.
Add the 500 ml thick coconut milk to the saucepan.
Add the teaspoon of tamarind extract. If using a lime, cut it in half and squeeze the juice of half of it into the saucepan.
Add 1 tsp white vinegar to the saucepan.
Bring this to the boil, turn it down till it is simmering and let it cook for 15 minutes.
Take the kitchen/absorbent paper off the fried fish and add it to the saucepan.
Let the fish cook in the sauce for 5 minutes. It is now ready to serve.