Shorshe Ilish (সর্ষে ইলিশ)

// SUPPLY LIST

// OPERATIONS

1. 1 If the ilish is frozen, thaw it overnight in the refrigerator, 10 to 12 hours, still sealed in its bag and standing in a dish to catch the drips, until the flesh gives under a finger but the fish is still cold to the core; never thaw it under running water or in a microwave, both of which turn this soft, fatty flesh cottony and leach out the fat that is the entire reason to cook this fish.
2. 2 Scale and gut the fish if the fishmonger has not, rinse it once whole under cold running water, then cut it crosswise with a heavy knife straight through the backbone into 2.5 cm (1 in) bone-in steaks, keeping the belly flaps attached and reserving the head and tail for another dish — you want about eight steaks, roughly 700 g of trimmed fish. Pat each steak dry with kitchen paper, cover and return them to the refrigerator while you make the paste, and do not rinse the cut pieces, because the slick you would wash down the drain is the hilsa fat you paid for.
3. 3 Put the yellow and black mustard seeds in a small bowl with 1/4 tsp of the salt and the 150 ml lukewarm water — hand-warm and no hotter than 40C/104F, never steaming — and soak for 30 minutes, until the seeds are plump and one crushes to a paste rather than a splinter under a fingernail; do not shorten this and do not use hot water, because dry seeds shear into bitter fragments in a grinder instead of breaking down to cream, and water above about 60C/140F kills the enzyme that makes mustard pungent in the first place.
4. 4 Drain the seeds, discarding the soaking water, and grind them with the 3 stemmed green chillies, 1/2 tsp of the salt and 3 tbsp of the cold water to a smooth, pale, creamy paste the colour of thin custard — on a stone sil-nora if you have one, otherwise in a small grinder jar in 10-second pulses with 20-second rests, scraping down each time; stop the moment the jar feels warm against the back of your hand, because friction heat, and mustard ground without salt and chilli, are the two things that turn the paste acrid.
5. 5 Press the paste through a fine sieve set over a bowl with the back of a ladle, working in up to 2 tbsp more cold water to help it through, and discard the tablespoon or so of gritty husk left behind — rub a little between finger and thumb and it should feel like cream, with no grit at all. Whisk the yoghurt smooth and stir it in off the heat (a West Bengali habit that rounds the edge off the mustard, and one Bangladeshi cooks leave out), then let the paste stand 10 minutes until it smells sharply of horseradish, which is the enzyme reaction finishing its work; use full-fat yoghurt only, as low-fat splits into grains in the steam.
6. 6 While the paste stands, lay the steaks in a single layer in a wide bowl, scatter over 1/2 tsp of the turmeric and 3/4 tsp of the salt, and turn them gently with your fingers so every face including the cut bone is coated; leave them 15 minutes at cool room temperature, around 20C/68F, until the surfaces look glossy and lightly beaded rather than wet, and handle them softly, because hilsa flesh is slack and breaks along the bone if you toss it about.
7. 7 Beat 3 tbsp of the mustard oil, the remaining 1/2 tsp turmeric and the last 1/2 tsp salt into the mustard paste until it is glossy and has the body of thin pouring custard, spoon it over the fish, turn each steak once to coat both faces, tuck 3 of the slit green chillies between the pieces, and marinate 20 minutes at cool room temperature and no longer than 30, refrigerating it if the kitchen is warm, because raw mustard and salt begin to firm and pickle the flesh before it ever sees heat.
8. 8 Arrange the steaks in one close, barely overlapping layer in a 1.5 litre stainless steel tiffin box or a 22 cm (9 in) heatproof dish at least 5 cm (2 in) deep, scrape every drop of marinade over them, and seal with a tight-fitting lid or two layers of foil crimped hard down the rim; stand the vessel on a trivet in a wide, deep pan or lidded wok holding 4 cm of water at a rolling boil (100C/212F), cover the pan and steam over medium heat for 16 to 18 minutes for 2.5 cm steaks, topping up with boiling water from a kettle if the pan threatens to run dry, since a dry pan scorches the base of the vessel and catches the sauce.
9. 9 Open the vessel with your face turned away, as the first breath of mustard steam is genuinely fierce, and check three things: the sauce has thickened to the consistency of pouring cream with a distinct slick of orange oil floating on top, the flesh parts from the backbone under light pressure from a knife tip, and it still looks glossy and only just translucent where it meets the bone; if it resists, reseal and steam 2 to 3 minutes more and no further, because ilish goes from perfect to dry and cottony inside about five minutes.
10. 10 Drizzle the remaining 2 tbsp mustard oil over the hot fish completely raw and unheated, drop in the last 2 slit green chillies, reseal the vessel and rest it off the heat for 10 minutes, until the sauce has settled and a clear orange film sits on the surface; do not smoke or heat that finishing oil the way you would for a jhol, because the volatile compounds you spent the last hour soaking, grinding and resting the mustard to build boil off the instant the oil passes its smoke point, leaving you with a bland yellow fish.
11. 11 Serve warm rather than scalding, two steaks per person with their share of sauce and orange oil spooned over a mound of plain steamed gobindobhog rice, with a whole raw green chilli and a wedge of gondhoraj lebu or lime alongside; in a full Bengali meal this comes after the bitter, dal and vegetable courses and before any meat, so bring it out then rather than first, warn the table that ilish is a very bony fish best eaten with fingers, and do not plan on leftovers, because reheating drives off the mustard and toughens the flesh.

// CLASSIFICATIONS