Make Mustard Paste: Grind mustard seeds, green chilies, garlic, and ginger into a smooth paste using a little water.
Marinate the Fish: Rub the fish pieces with turmeric and salt. Let it sit for 10 minutes.
Fry the Fish: Heat oil and fry the fish pieces until light golden. Remove and set aside.
Prepare the Curry Base: In mustard oil, saute onions, curry leaves, and green chilies until golden.
Add Mustard Paste: Lower the heat and stir in the mustard paste. Cook for 2–3 minutes.
Add Spices and Tomatoes: Add turmeric, coriander, garam masala, and a little salt. Stir for 1–2 minutes.
Add a splash of water, then chopped tomatoes. Cook until oil separates.
Add the Fish: Gently mix in fried fish. Add 1 cup hot water and bring to a gentle boil.
Simmer uncovered for 6–8 minutes. Avoid over-stirring.
Finish and Serve: Garnish with fresh coriander (optional).
Serve hot with plain rice.