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1
Rub the prawns with half the turmeric and a pinch of salt, so the turmeric begins to season and faintly firm the flesh before the pan is even lit. Set a wide, heavy pan over high heat and bring the mustard oil to smoking point, then reduce to medium-high — smoking the oil briefly drives off its raw pungency, leaving behind a mellow, nutty heat that carries the whole dish. Add the prawns in a single layer and fry, turning once, for no more than 90 seconds per side; the surface is sealed when it shows a pale gold crust and releases cleanly from the pan, with the edges opaque but the centre still translucent.
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2
In the same pan and oil, add the bay leaves, cardamom, cloves and cinnamon and fry over medium heat until the spices swell and the cardamom skins blister, so the fat draws out their volatile oils and the kitchen fills with a sweet, woody fragrance. This infused oil is the aromatic base into which everything else will cook, so give it a full minute rather than rushing to the next step.
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3
Add the onion paste and cook, stirring frequently, until it is dry and pale golden — the raw, sharp smell gives way to something mellow and sweet, and the paste no longer sticks in wet clumps, which tells you the moisture has cooked out and the onion will not thin the gravy later. Add the ginger paste and the remaining turmeric and cook for a further minute until the fat begins to separate at the edges of the paste, a sign that the aromatics are cooked through and no longer taste raw.
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4
Pour in the thin coconut milk (second extract) and add the slit green chilli and sugar, then stir to combine. This is where the dish lives or dies: the thin extract enters first because it can withstand a longer simmer without breaking, whereas adding the thick extract at this stage would subject it to extended heat, splitting the fat from the solids and producing a greasy, broken gravy rather than a creamy one. Bring the pan to a gentle simmer, uncovered.
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5
Return the partly cooked prawns to the pan and simmer uncovered over low-medium heat for 3 to 4 minutes, turning them once so the residual heat finishes the centres without the exterior turning rubbery. The prawns are done when they are uniformly pink and just firm to a light press — they should yield with the slightest resistance, not spring back hard.
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6
Reduce the heat to its lowest setting, pour in the thick coconut milk (first extract) and stir gently in one direction, so the rich cream folds into the gravy smoothly rather than separating where it meets the hotter liquid. Do not cover the pan from this point — trapped steam raises the temperature unevenly and curdles the coconut milk. Warm through for 1 to 2 minutes until the gravy is uniformly pale gold and glossy and coats the back of a spoon in a light, creamy veil without any visible oily pooling.
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7
Remove from the heat, add a pinch of garam masala and taste for salt. The garam masala goes in off the heat so its fragrance stays bright and top-noted rather than cooking away into the background, consistent with the dish's restrained spice character. Serve immediately over gobindobhog or basmati rice.