Mix the ginger with the turmeric, garlic and curry paste in a large bowl. Stir in the yogurt until well combined, then add the fish and toss to coat 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.
Add 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. Season to taste and stir in the coriander before serving.