Take the chopped chicken and coat it with the cumin, ground coriander and turmeric then leave it to marinate in the fridge while you make the sauce.
To make the sauce, fry the roughly chopped onions with the garlic and green chili in a large pan until browned.
Add the water to the onion mixture and simmer this for around 20 minutes.
Put the plum tomatoes in a food processor and puree.
Heat another large pan and gently fry the ground coriander, cumin and turmeric in a splash of oil for about a minute.
Add the sauce to this pan and simmer for around 10 minutes.
Blend the onion mixture in the food processor and add it to the spiced tomato sauce. Give it a stir and simmer for 20 minutes.
Fry the marinated chicken in oil and stir continuously.
After a few minutes, turn down the heat and add the other half of the chopped onion, the red pepper and chilies. Stir this until the onions and pepper soften.
Add the earlier prepared sauce to the cooked chicken and simmer for around 10-20 minutes.
Just before serving, stir in the garam masala and chopped coriander leaves.