To make the spice paste, grind all the ingredients with a little water till you have a smooth paste.
Heat oil in a pan and cook the onion till soft and translucent. Now add the curry leaves and the spice paste.
Cook for 3-4 mins on low heat till it loses the smell of raw garlic and turmeric. Now add the chopped tomatoes, fresh chillies and salt. Cover and cook for 2 mins till the tomatoes are squishy.
Pour in the thin coconut milk and bring to a gentle simmer. Slip in the fish slices and cook for 10 mins till they are opaque and done. Now add the vinegar and thick coconut milk, check the seasoning and simmer for a further 4-5 min. Serve with steamed rice.