Cut each fish fillet into two pieces and very thoroughly pat them dry.
Heat the oil in a heavy-based frying pan over low heat and fry the onion until softened and lightly browned. Add the ginger, garlic and spices and stir for 2 minutes. Add the yoghurt and green chilli and bring to the boil. Season with salt, then cover and simmer for 10 minutes. Slide in the pieces of fish and continu