Marinate the fish steaks with salt and turmeric powder. Set aside for 30 minutes.
Dry roast coriander seeds, cumin seeds, mustard seeds, and fenugreek seeds in a pan until aromatic.
Grind them into a coarse powder using a mortar and pestle (or a mixer for finer powder).
In a kadai or deep pan, heat mustard oil until it just starts to smoke.
Reduce the heat slightly and fry the marinated fish pieces on both sides until light golden. Remove and set aside.
In the same oil, add 1 tsp fenugreek seeds and let them crackle.
Add the chopped onions and fry until golden brown.
Stir in the ginger-garlic paste and cook for 1–2 minutes.
Add the roasted ground spice powder and stir well.
Add the turmeric and red chili powder mixed with a little water. Fry until the raw aroma disappears.
Add chopped tomatoes and cook until soft and the oil separates. Splash in a little water as needed to prevent burning. Also, add the grounded dry fenugreek leaves (kasuri methi).
Pour in 300 ml water, bring the mixture to a boil, and let it simmer for 2–3 minutes.
Gently add the fried fish pieces back into the pot. Cover and cook for 10 minutes, allowing the fish to absorb the curry flavors.
Garnish with fresh coriander leaves and serve hot with plain rice or chapati.