Pound the garlic, chillies and ginger with a pestle and mortar until you have a smooth paste.
Heat the oil in a large nonstick wok or saucepan over a medium heat. Add the paste and stir-fry for 2–3 minutes, then add the onion and turmeric. Stir-fry for a further 2–3 minutes until fragrant.
Stir in the coconut milk, measured water and the potatoes. Bring to the boil, reduce the heat to low 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 and simmer gently for 6–8 minutes until the fish is cooked through. Remove from the heat and garnish with chopped coriander. Serve hot with steamed white rice.