Preheat the oven to 180ºc/Gas 4.
Heat the oil in a saucepan and fry the lamb until lightly browned all over. Remove from the heat and set aside. You may need to do this in batches.
In the same pan, add the onions and cook over a medium heat for about 6mins until they're softened, stirring regularly. Turn down the heat and add the garlic, ginger, chilli, curry paste and tinned tomatoes, stirring frequently.
Remove from the heat and blend until you have a smooth paste.
Pour the mixture into a large casserole dish, add the lamb, potatoes, water, bay leaves, salt and sugar. Place in the middle of the oven and cook for 1hr 30mins or until the lamb is tender and the sauce has thickened. After an hour, remove, stir and place back in the oven to continue cooking.
Add the quartered tomatoes and spinach and cook for a further 10mins until the tomatoes have soften.
Remove and serve with rice and plain naan.