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1
Toss the prawns with 1 tsp turmeric and 1 tsp salt and set aside for 10 minutes, so the salt draws moisture to the surface and the turmeric begins to penetrate the flesh rather than sitting as a dry coat. The flesh will turn a pale ochre and feel lightly tacky to the touch.
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2
Heat the mustard oil in a wide, heavy pan over a medium-high flame until it just begins to smoke and loses its raw sharpness — this smoking-off is what turns the oil from acrid and nose-catching to mellow and round, the base note the whole dish rests on. Drop in the two whole dried red chillies and let them bloom for 20–30 seconds until they darken and the oil carries a faint smokiness.
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3
Add the marinated prawns in a single layer and fry, turning once, until the shells are bright orange and the flesh is just opaque to approximately 70% of its depth — the centre should still look glassy when a prawn is pressed, so that the residual heat of the sauce finishes them gently rather than pushing them past tender into rubbery. This is where the dish lives or dies: a prawn fully cooked at this stage will be hard and dry by the time it returns to the sauce. Remove immediately and set aside.
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4
In the same oil, over a medium flame, fry the sliced onion until soft and translucent with pale golden edges, so the onion sweetens and loses the sharpness that would otherwise cut through the delicate coconut base. Add the ginger and slit green chillies; stir for one minute until the raw ginger smell gives way to a sweeter, roasted note.
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5
Add ½ tsp turmeric and salt, stir to coat the onion, then pour in the thin coconut milk (second press), so the spices bloom in the oil-slicked onion before the liquid arrives and carry their colour evenly through the sauce. Bring to a gentle simmer and cook, uncovered, for 4–5 minutes until the sauce is faintly thickened and pale yellow — the surface should show a light, rolling movement, not a hard boil.
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6
Return the prawns to the pan and cook on a low flame for 3–4 minutes, turning once, so the flesh finishes through in the sauce and takes on its flavour rather than simply reheating in it. The prawns are ready when the flesh yields without resistance when pressed and is fully opaque.
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7
Remove the pan from the heat or reduce to the lowest possible flame before pouring in the thick coconut milk (first press), stirring gently to combine, because the first press will break into greasy pools and turn grainy the moment it reaches a simmer — off the heat it stays silken and holds the sauce together. The sauce should remain thin and pale yellow. Taste for salt.
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8
Serve immediately over steamed basmati or gobindobhog rice. Ladle the sauce over the rice first, then place the prawns on top, and set a lemon wedge and a raw green chilli alongside each portion.