Clean, wash and cut the fish into fillets (do not remove the bones).
Grind the coconut with turmeric and cumin seeds. Extract the milk twice (add a little water the second time).
Heat the ghee or oil in a pan. Add the chopped onions, ginger and ground coconut. Fry for a minute or two.
Add the tomatoes and chillies and fry over a very low heat (do not allow any of the ingredients to brown). Cook for 5 minutes, stirring frequently.
Add the second extract of coconut milk and stir. When it reaches simmering point add the fish and cook gently uncovered for 10 minutes.
Add the thick coconut milk. Stir gently.
Add salt and vinegar, simmer over low heat for a few minutes and remove.
Add the thinly sliced onion and tomato. Sprinkle with lemon juice.