Cut the fish into chunks about 1½ - 2 inches square. Sprinkle with the salt and the ¼ tsp turmeric. Mix gently and set aside while you make the curry. Keep all the curry ingredients ready near the hob before you proceed.
In a medium frying pan set over medium heat, add the oil then add the mustard seeds. When they splutter add the fenugreek seeds, stir and fry for a few seconds until they go a shade darker, then add the curry leaves and stir.
Add the onions, garlic, ginger & green chillies and fry on gentle heat for about 5 - 10 minutes until the onion looks golden.
Now add the tomato puree, salt to taste, the remaining ⅛ tsp turmeric powder, the two chilli powders and fry for another 5 -10 minutes until oil oozes from the periphery and the mixture looks fried.
Add 1 cup of water and bring to the boil. Lower heat and simmer, then gently add the fish pieces. Simmer uncovered, until the fish pieces are done. Add lemon or lime juice to taste and serve with fresh steamed rice.