Why this recipe works
The heat here is made on demand. Whole mustard seeds are almost odourless; ground with water, an enzyme meets a compound to create the sharp, nose-tingling molecule (allyl isothiocyanate). But that enzyme is destroyed by heat, so the paste is added late and barely cooked, and over-grinding makes it bitter. It is a genuinely light curry: mustard paste, a little mustard oil, and lean, omega-3-rich fish.
Ingredients
Method
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1
Soak the mustard seeds, then grind with green chilli, a little salt and water to a smooth paste.
Why: grinding with water triggers the enzyme reaction that makes mustard pungent, and a smidgen of salt and chilli keeps the paste from turning bitter.
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2
Rub the fish with turmeric and salt and lightly fry in mustard oil, then set aside.
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3
In the same oil, temper nigella, add the mustard paste with a little water and warm it gently.
Do not overcook the paste: high, prolonged heat destroys the pungent compound and turns the mustard flat and bitter, so keep it a gentle simmer.
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4
Slip in the fish and simmer briefly until just cooked and coated.
Gentle heat: fish overcooks in moments and turns rubbery, so a few minutes on low is all it needs.
An honest word
Shorshe maach is a genuinely light, healthy fish curry: no cream, modest oil, a sharp mustard sauce and lean, omega-3-rich fish. There is very little to caveat beyond going easy on the mustard oil and grinding the paste smoothly to avoid bitterness. Served with plain rice, it is one of the cleaner, more nourishing fish dishes you can make.