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1
Heat the oil in a wide, heavy pan over medium heat, then add the mustard seeds and wait until they begin to pop — the popping tells you the oil is hot enough to wake the seeds fully, releasing their nutty fragrance rather than leaving them raw and bitter. Add the curry leaves immediately after; they will spatter, so stand back. Add the onions and cook, stirring regularly, until they are deep golden and beginning to catch at the edges, because only at that colour do they carry the sweetness and body the base needs.
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2
Add the ground coriander, turmeric, and chilli powder, and stir for one minute so the spices toast in the residual oil and lose their raw, floury edge before any liquid arrives. Add the tomatoes and cook over medium heat, pressing and stirring as they collapse, until the tomato and onion have merged entirely and the mixture has darkened several shades — the colour shift is your sign that the acidity has cooked out and the flavours have knitted together.
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3
Add the tamarind water and continue cooking, stirring frequently, for 20–25 minutes in total from the addition of the tomatoes — this is where the dish lives or dies, because the base must reduce to a thick, concentrated paste and the oil must visibly separate and pool at the edges before any further liquid is introduced; if you add water too soon, no amount of extra simmering will recover the depth of flavour that only this reduction can build. Watch for the paste pulling cleanly away from the sides of the pan and the oil forming bright orange puddles at the rim — those are your cues, not the clock.
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4
Pour in approximately 400–500 ml of water or coconut milk and stir to lift the paste evenly into the liquid, so no concentrated pockets remain at the bottom of the pan. Taste for salt and bring the sauce to a steady simmer before adding the fish, because fish dropped into a cold or under-seasoned sauce will turn dull and absorb the wrong flavours.
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5
Lay the fish pieces into the simmering sauce in a single layer and poach, without stirring, for 8–12 minutes — no more — because turning the pieces breaks the flesh apart before it has the chance to firm and hold. The fish is done when the flesh is opaque throughout and yields without resistance when pressed lightly; pull the pan from the heat at that exact moment, as even a minute or two beyond it will leave the fish dry and fibrous.