Rinse the fish pieces and pat them dry. Rub them with a pinch of salt and 1/2 teaspoon of ground turmeric. Set aside for 10 minutes.
In a large pan, heat the mustard oil over medium heat until it starts to smoke slightly.
Add the finely chopped onion and sauté until golden brown.
Add the minced garlic, minced ginger, and slit green chilies. Cook for another 2-3 minutes until fragrant.
Add the chopped tomatoes and cook until they are soft and the oil starts to separate from the mixture.
Stir in the remaining ground turmeric, ground cumin, ground coriander, red chili powder, and ground mustard seeds (if using). Cook for another minute.
Pour in the water and bring the mixture to a gentle simmer. Add salt to taste.
Gently add the fish pieces to the pan. Cover and let it simmer for about 10-15 minutes, or until the fish is cooked through and the flavors have melded together. Be careful not to stir too much to avoid breaking the fish pieces.
Transfer the Hilsa Fish Curry to a serving dish. Garnish with freshly chopped cilantro.
Enjoy your meal!