Preheat the oven to 200ºC/Gas Mark 6. Sprinkle the fish with salt inside and out and set aside for 20 minutes. Meanwhile, heat the oil in a large pan, add the onion, sprinkle with salt and fry until it begins to brown.
Lower the heat, add the curry paste and curry leaves and continue to fry gently for another 2–3 minutes, stirring occasionally. Meanwhile, slice the aubergine lengthways
into 4 wedges, then each wedge across into slices 1cm thick.
Add the coconut milk, 500ml water, chillies, okra, aubergine, tomatoes, tamarind water and 2 teaspoons salt and simmer for 15 minutes.
Put the mackerel side by side in a deep roasting tin in which they fit quite snugly. Pour over the curry sauce (it should almost cover the fish), cover tightly with foil and transfer to the oven.
Bake for 30–35 minutes, until the fish are cooked through. Remove, season to taste with more salt if necessary, and garnish with a sprinkling of coriander leaves.