Bengali Shorshe Ilish (Hilsa in Mustard Sauce)

Ingredients

• 1 1/2 lb bone-in hilsa steaks, about 6 pieces, thawed if frozen
• 1 tsp ground turmeric, divided
• 1 1/2 tsp fine sea salt, divided, plus more to taste
• 3 tbsp yellow mustard seeds
• 1 tbsp black or brown mustard seeds
• 5 fresh green chiles, divided
• 5 tbsp culinary mustard oil, divided
• 1/2 tsp nigella seeds, optional
• 3/4 cup warm water, plus cool water for soaking and grinding
• Steamed short- or medium-grain white rice, for serving

Method

1. 1.Put the yellow and black mustard seeds in a small bowl, cover with cool water and soak 30 minutes. Drain thoroughly. Keep the water and blender jar cool rather than warm so friction does not turn the paste bitter.
2. 2.Pat the hilsa steaks dry without pulling at the flesh. Rub them gently with 1/2 teaspoon turmeric and 1/2 teaspoon salt and set aside for 15 minutes while making the paste.
3. 3.Blend the drained mustard seeds with 2 green chiles, 1/2 teaspoon salt and 3 tablespoons cool water in short pulses. Scrape the jar and add water 1 teaspoon at a time until the paste is smooth and pourable, never letting the jar become hot.
4. 4.Taste only a tiny dab of the mustard paste. It should be powerfully pungent but not acrid. For an especially smooth sauce, push it through a fine sieve and discard the coarse hulls rather than continuing to grind them.
5. 5.Heat 4 tablespoons culinary mustard oil in a wide lidded skillet over medium until shimmering. If using nigella, add it and cook 20 to 30 seconds, just until fragrant; do not let the seeds blacken.
6. 6.Lower the heat. Stir in the mustard paste, remaining 1/2 teaspoon turmeric and 3/4 cup warm water. Add 1/2 teaspoon salt and 2 slit green chiles, then bring only to a gentle simmer for 3 minutes. The raw edge should soften while the sauce stays yellow and fluid.
7. 7.Slide the hilsa steaks into the sauce in one layer and spoon gravy over their tops. Cover and cook over low heat for 5 minutes, keeping the sauce at a lazy simmer rather than a hard boil.
8. 8.Turn each steak once with a thin spatula and support it with a spoon. Cover and cook 4 to 6 minutes more. Checkpoint: the flesh should be opaque and moist, separate at the outer edge under gentle pressure and register 145°F at the thickest part.
9. 9.Remove the pan from the heat. Add the remaining green chile, slit lengthwise, and drizzle over the final 1 tablespoon culinary mustard oil. Cover and rest 5 minutes so the sharp aroma settles into the sauce without further cooking the fish.
10. 10.Taste the gravy around the fish and adjust salt with a light hand. Serve one steak per diner with plenty of hot rice and explain the fine bones before anyone begins eating.
11. 11.Cool leftovers within 2 hours and refrigerate in a shallow covered container for no more than 1 day. Reheat once, covered and very gently, adding a spoonful of water so the sauce loosens without breaking the fish.