For the Sri Lankan curry powder, grind coriander seeds, cumin seeds, fenugreek seeds, cinnamon, cloves, cardamom, dried curry leaves and pepper corns in a grinder to a fine powder. Store it in a screw top jar.
Marinate the fish slices with salt and lemon juice and keep aside for 1/2 an hour
Fry the marinated fish pieces in mustard oil till they turn light brown in colour.
Heat oil in a large, shallow pan and add the onion, garlic and curry leaves to it.
Fry gently for 7-10 minutes until the onions are soft and lightly golden in colour.
Add the turmeric, chilli powder and Sri Lankan curry powder and fry it for 1-2 minutes.
Add the tomatoes, coconut milk and 1 tsp of salt and let it simmer for 5 mins.
Add the fish to the pan and spoon the gravy over the fish.