Place the fish fillets on a plate, sprinkle over half the ground turmeric and half the salt and rub into the fillets. Leave for a minimum of 10 minutes but no longer than 30 minutes.
In a heavy-based frying pan (skillet), heat the oil over a medium– high heat.
Add the sliced onions to the pan and fry gently, stirring occasionally, until golden brown and caramelized. Using a slotted spoon, remove the onions from the pan, leaving as much of the oil in the pan as possible to cook the other ingredients, and place on a plate to drain. Spread the onions across the plate so they crisp as they cool.
You should have enough oil left in the pan to fry the fish; if not, add another 1 tbsp vegetable oil. In the same pan, flash-fry the fish fillets for 20–30 seconds on both sides to seal. Do not allow the fish to cook.
Remove the fish from the pan and set aside on a plate.
Keeping the heat at medium–high, add the garlic and ginger pastes to the pan and cook, stirring, for 1 minute. Add the remaining ground turmeric and the chilli powder. If the pastes stick to the base of the pan, sprinkle over some water. Add the tomato puree, 4 tbsp warm water, the remaining salt and the fried onions, then cook for few minutes until the oil has seeped to the edges of the pan.
Return the fish fillets to the pan and cook for a further 2 minutes. Add the coconut milk, then immediately remove the pan from the heat and carefully turn each fillet over. Taste the coconut milk and adjust the seasoning with sugar or salt as necessary. Before serving, garnish with sliced green chillis and chopped coriander.