Pound the garlic, chillies and ginger in a small mortar with a pestle until you have a smooth paste.
Heat the oil in a large nonstick wok or saucepan and add the garlic and chilli mixture. Stir-fry for 2–3 minutes, then add the onion and turmeric. Stir-fry for a further 2–3 minutes, then stir in the coconut milk, measured water and potatoes.
Bring to the boil, reduce the heat and simmer gently for 10–12 minutes, stirring occasionally.
Season the fish with salt and add to the pan with the tomatoes. Bring the mixture back to the boil, reduce the heat and simmer gently for 6–8 minutes or until the fish is cooked through. Garnish with chopped fresh coriander just before serving.