Marinate fish pieces with turmeric and salt for 10 minutes.
Heat mustard oil in a heavy-bottomed pan and fry mustard seeds and nigella seeds until they splutter.
Add onion paste, ginger paste, garlic paste, and tomato paste; cook until oil separates from the mixture.
Add turmeric, coriander powder, cayenne, and garam masala; mix well and cook for 2 minutes.
Add 2 cups water and bring to a simmer, then gently add marinated fish pieces without stirring.
Cook for 8-10 minutes until fish is tender, then add lemon juice and green chilies.
Simmer for 2-3 minutes more and garnish with chopped cilantro before serving.