2 teaspoons turmeric
1 tablespoon vegetable oil
2 medium onions, halved and cut into fine half-moons
2 long red chillies
4cm piece fresh ginger
Pinch ground cumin
1 x 400ml tin coconut milk
1-2 tablespoons concentrated tamarind
1 tablespoon liquid fish stock
Cut the fish into bite-sized chunks, put them into a large bowl, and rub with a little salt and 1 teaspoon turmeric. Heat the oil in a large, shallow pan and peel and tip in your fine half-moons of onion; sprinkle them with a little salt to stop them browning and then cook, stirring, until they've softened; this should take scarcely 5 minutes.
Cut the whole, unseeded chillies into thin slices across (although if you really don't want this at all hot, you can deseed and then just chop them) and then toss them into the pan of softened onions. Peel the ginger and slice it, then cut the slices into straw-like strips and add them too, along with the remaining teaspoon of turmeric and the cumin. Fry them with the onions for a few minutes.
Pour the tin of coconut milk into a measuring jug and add a tablespoon of tamarind paste and the fish stock, using boiling water from the kettle to bring the liquid up to the litre mark. Pour it into the pan, stirring it in to make the delicate curry sauce. Taste and add more tamarind paste if you want to. And actually you can do all this hours in advance if this helps.
When you are absolutely ready to eat, add the fish to the hot sauce and heat for a couple of minute until it's cooked through but still tender.