Season the fish fillets with a little salt and turmeric and set aside.
Toast the spices for the masala on low heat for a minute at most and grind to a fine powder.
Heat the oil in a pan on medium heat and add the tempering of mustard, cumin and fenugreek seeds. Allow them to crackle and take care not to burn them or they'll turn bitter.
Tip in the chopped onion and a few curry leaves and sauté on low flame till the onion is soft and very lightly golden.
Add the garlic and ginger pastes and continue to sauté, (scraping the pan intermittently to prevent burning) till the smell of raw garlic and ginger disappears and the mixture is golden.
Now add the chilli powder and coriander powder mixed in a tbsp of water. Cook for 20 seconds before pouring in the tomato puree. Add salt and green chillies and let the tomatoes cook till the oil starts separating on the sides
Put in the tamarind paste and enough 400 ml water. Bring to a boil, reduce the heat and simmer for 5-7 min till the sauce is cohesive and cooked. Add more water if needed.
Slide in the fish and cook for 4-5 minutes till they're opaque. do not stir too much else the fillets may break.
Sprinkle the ground masala and remaining curry leaves over the curry, gently shake the pan to incorporate them, cook for 20 seconds turn off the heat. Serve hot with steamed rice.