Coat the chicken pieces with the coriander, cumin and turmeric mix, then leave to marinate in the fridge while making the sauce.
Take half of the onion and chop it roughly. Heat 1 tblsp of the oil in a saucepan and fry the onion, garlic and green chilli, until browned - about 5 to 10 minutes.
Add the water to the onion mixture and simmer for around 15 minutes. Add more water if it begins to dry out.
While the onion mixture is simmering, heat a splash of oil in a wok or fry-pan and gently fry the coriander, cumin and turmeric for about a minute. Add the tin of tomatoes to this mixture and simmer for 10 minutes.
After 10 minutes, mash the tomato mixture and add it to the onion mixture in the saucepan. Stir and then simmer for a further 20 minutes.
Slice the other half of the onion into thin strips and then in a wok (wipe out the one used to make the tomato mixture if you like) heat 1 tblsp of oil and fry the marinated chicken, stirring continuously.
When the chicken is sealed after a few minutes, add the sliced onion, the red pepper and chillis (if using). Stir this until the onions and peppers soften and the chicken is cooked.
Add the sauce to the chicken and peppers and simmer for around 10 to 20 minutes.
Just before serving, stir in the garam masala and chopped coriander leaves and then serve with basmati rice and/or naan bread.