Marinate Fish: Pat hilsa pieces dry. Rub with 1 tsp turmeric and 1 tsp salt. Rest 10 minutes.
Prepare Mustard Paste: Soak mustard seeds in warm water 10 minutes, drain. Grind with chopped chilies and 2 tbsp water to a smooth paste; adjust consistency.
Heat Oil & Temper: Heat mustard oil until it just smokes, reduce to medium. Add nigella seeds; let pop briefly.
Saute Aromatics: Add slit chilies and ½ tsp turmeric; stir 5 seconds.
Cook Mustard Paste: Lower heat, add mustard paste; stir 1 minute.
Build Gravy: Pour in ½ cup water, season with ½ tsp salt. Bring to gentle simmer.
Add Fish: Gently place hilsa pieces in gravy, skin-side down. Cover; cook low 5–6 minutes.
Flip & Finish: Turn fish carefully, spoon gravy over. Simmer 3–4 more minutes until cooked and gravy slightly thickened.
Rest & Garnish: Off heat, let rest 2 minutes. Garnish with a slit green chili.