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1
Soak the yellow and black mustard seeds together in cold water for 30 minutes, then drain — soaking softens the seeds so the blender reduces them to a genuinely smooth paste rather than a speckled, gritty one. Transfer to a blender with the green chillies, turmeric, and a small amount of water, then blend to a fine paste, adding water gradually. Taste the paste: it should be sharp and pungent but not aggressively bitter, because aggressive bitterness indicates old seeds or insufficient soaking and the paste cannot be corrected at a later stage.
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2
Combine the hilsa steaks with the mustard paste and the additional turmeric, turning the steaks so every cut surface is thoroughly coated, since the paste must be in direct contact with the flesh to perfume it rather than simply sitting on the outside. Set aside to marinate briefly while the pan is prepared.
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3
Arrange the marinated steaks in a single layer in a wide, heavy pan together with all the marinade paste, then drizzle the raw mustard oil evenly over the fish — this is where the dish lives or dies: no additional liquid is added, and the fish must steam entirely in its own released juices and the marinade, which together form the sauce. Cover the pan tightly, because any breach of the seal dissipates the steam and dries the fish before the sauce can come together.
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4
Cook over a low-to-moderate heat, covered and undisturbed, until the flesh yields in large, moist, oil-rich flakes that separate cleanly from the bone, checking gently only near the end so you do not break the seal prematurely and lose the gathered steam. Overcooked hilsa surrenders its characteristic richness and becomes dry, so the moment the flesh releases from the bone with gentle pressure, the pan must come off the heat.