Heat the oil in a saucepan and fry the chicken 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 6 mins 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 an oven proof casserole dish, return the chicken, potatoes, water (see tip below), bay leaves, salt and sugar. Bring the sauce to a boil and then reduce and simmer for 45 mins or until the chicken is cooked through and the sauce has thickened.
In the meantime, cook the rice according to the packets instructions.
Just before dishing up, stir in the spinach and serve with rice and plain naan.