Place the onion, garlic, chillies, cumin, ground coriander, turmeric, coriander leaves and measured water in a food processor and blend to a smooth paste.
Heat the oil in a large frying pan over a high heat. Add the curry leaves and stir-fry for 20–30 seconds. Now add the blended paste and cook, stirring, over a high heat for 3–4 minutes until fragrant. Reduce the heat, pour in the coconut milk and simmer gently, uncovered, for 20 minutes.
Add the fish to the pan in a single layer and bring back to the boil. Reduce the heat and simmer gently for 5–6 minutes until the fish is just cooked through. Season and remove from the heat. Garnish with coriander leaves and serve with steamed basmati rice.