Heat the oven to 170°C/150°C fan/Gas 4. Put a casserole, or similar, over a medium heat. Add two tablespoons of oil, the onions and a big pinch
of salt. Cook gently for 15–20 minutes, stirring occasionally, until soft and lightly golden – we’re not aiming for fully browned in this instance. Meanwhile, measure all the dry spices, except the turmeric, into a dry pan and toast over a low heat for 3–4 minutes until fragrant. Decant into
a spice grinder or pestle and mortar and reduce to a powder, then stir in the turmeric.
Add the curry leaves, bay leaves and garlic to the onion and cook for 2 minutes more, then add 2 heaped tablespoons of the ground spices and a tablespoon or two of water. (You will have some spice powder left over – potentially to be added later, but otherwise reserve for a generic
‘curry powder’ moment. The mix works well, for example, in the curry leaf mussels on page 150).
Add two thirds of the tomatoes to the pan and mix with the onions. Sprinkle with the sugar, dot with the butter and place in the oven (without a
lid) for 1 hour 20 minutes. Give it a shuffle after 30 minutes to ensure the onions and tomatoes are mingling, then 20 minutes later add the
remaining tomatoes and carefully mix. Return to the oven for the final (pre-fish) 30 minutes.
Stir another tablespoon of the ground spices into the tomatoes, then sit the fish skin-side up, pushing them 1–2cm (½–¾in) below the surface
so that they part-poach, part-bake. Return to the oven, still uncovered, for 12–14 minutes so the fish is just cooked – flaking but still succulent.
While the curry is cooking, make the turmeric yoghurt. Heat the rapeseed oil, add the garlic and cook gently for 1–2 minutes without burning.
Remove from the heat, add the turmeric, stir and allow to cool. Just before eating, add to the yoghurt with a pinch of salt and stir vigorously
until bright yellow.
Peel the skin from the fish and serve with ladles of tomatoes alongside brown rice and turmeric yoghurt, with fresh green chilli scattered to taste.