Keralan Fish Curry with Mango and Tamarind

You may be unfamiliar, as I was for a while, with South Indian cuisine. For some reason, it is extremely different from Northern Indian food, being, in my opinion, more similar to Thai curries; coconut milk based, with lighter, but more emphatic sweet and sour flavours. The dish I based this particular curry on is called 'meen manga' and is soured from the flavour of underripe mangoes, but using them would hurt my soul, not least because I hate the grassy flavour of an unripe mango. Instead, I use a ripe mango, in fact here so ripe that instead of chopping into neat cubes although normally I would cut two slices on either side of the stone and use a glass to push the flesh from each slice from the skin, I simply mangled the chopped fruit, getting juice, pulp and flecks of flesh. To get the sour element, I used tamarind concentrate, but you could use the block you have to dissolve in water, but it's a bit more of a pain. If you opted for that camp, you'll need to probably triple the mass as it's not concentrated.
1)This step is common in almost all curries, (but this comes from a person who makes a spiced stew from leftover meat and calls it curry) you make a base with one sliced onion, and sauté it with whatever flavours are appropriate. Here, you need 3cm finely shredded ginger, 3 crushed garlic cloves and 2 green chillies, finely chopped. It's up to you if you leave the seeds in or not.
2)Whilst the onions are softening, coat 4 small white fish fillets (450g cod, hake, basa etc) that have been chunked thickly in 1 1/2 tsp ground turmeric and the same of cumin, plus some salt. You may want to use washing up gloves for this to keep your hands from looking like your liver's given up. Leave to marinate whilst you prepare the rest of the curry.
3)Add very simple spices to the onions- 2 tsp ground coriander, a pinch or more (if you dare) of chilli flakes and a small handful of curry leaves.
4)Make a sauce with a 400ml tin coconut milk, 1 2/3 (1 tbsp and 2 tsp) tamarind concentrate, and bring it up to a litre with fish stock. Add to the onions, and simmer it but don't let it come to a rolling boil or the coconut milk may separate. It probably will anyway to be frank but no need to shatter the emulsion. Once simmering add 3 small or 2 large mangoes, prepared as above.Add your fish and 2 mangoes prepared as above. Once they're done squeeze any juice or pulp from the stones and then, as your cook's treat, scrape the buttery flesh from the stone with your teeth. Let it simmer for about half an hour. At this stage you can turn the heat off and reheat the curry as and when you need it, adding the fish last minute.
5)Before you serve, add the fish and cook until just tender, about 4 minutes.
5)Serve over tender rice and the finely chopped coriander leaves. I added some frozen peas and green beans to this, but looking back I really don't think it needed it.