Curry Sauce: Heat the oil in a heavy frypan. Add onion and stir for 2-3 minutes on high heat. Add ginger, garlic and green chilli. Turn heat onto low. Cook for 15 minutes occasionally stirring to avoid browning/burning.
Add turmeric, cumin and coriander and cook further for 5 minutes.
Take off the heat to cool. Pour 125ml cold water in a blender. Add sauce from the frypan, blend till very smooth. Stir in tomato paste.
Pour the mixture back into the pan. As you prepare the rest of the balti ingredients, stir occasionally for approximately 30 minutes over very low heat. Sauce should darken in colour to orange-brown.
Balti: Heat 1 tablespoon of the oil in a wok or balti pan over high heat. Add the capsicums and stir fry until edges slightly browned. Remove the capsicums from the wok and set aside.
Put a little more oil into the wok and heat through then add some of the chicken pieces and stir fry until they are sealed and have turned white. Remove the chicken from the wok and set aside. Stir fry the chicken in batches to avoid stewing.
Turn heat down to medium. Add the remaining oil to the wok. Add the onion, chillies and cumin seeds. Stir fry until the onion is translucent. Do not allow to brown. Add the paprika, turmeric and cinnamon. Stir fry for 30 seconds.
Return the chicken pieces to the wok. Mix in the curry sauce from the other pan. Add tomato paste and salt.
Bring to a simmer, then turn heat to low and cook for 15 minutes. Stir in the capsicum and garam masala. Continue cooking for 15 minutes until the chicken is done. Sprinkle in hot water if the sauce is becoming too thick.