Mix together the ginger, turmeric, garlic and curry paste in a bowl. Stir in the yogurt until combined. Cut the fish into large pieces and add to the bowl, stirring until coated in the spice mixture.
Heat the oil in a large saucepan and gently fry the onion, cinnamon, sugar and bay leaves until the onion is soft. Add the tomatoes, stock and potatoes and bring to the boil. Cook, uncovered, for about 20 minutes until the potatoes are tender and the sauce has thickened.
Tip in the fish and spicy yogurt and reduce the heat to its lowest setting. Cook gently for about 10 minutes or until the fish is cooked through. Check the seasoning and stir in the coriander to serve.