Cut the chicken into cubes. Mix the ingredients together in a large bowl, cumin, coriander, chilli powder, salt, ginger/garlic paste. Add the chicken and coat well with the mixture. Refrigerate for 30 minutes
Heat 2 tablespoon oil in a large pan, on high heat. Add half the chicken and allow it to brown on all sides. Repeat with other half of the chicken. Chicken doesn't have to be fully cooked as it will cook further in the sauce. Cover with foil and set aside
Reduce the heat to a low setting. Add one tablespoon of butter to the same pan. Chop half the onion finely and add to the pan. Fry until translucent. Add the garlic and fry for another minute
Add the can of tomatoes and the chicken stock. Simmer for 15 minutes or until the sauce thickens
Add the spices, cumin, coriander, turmeric, garam masala and chilli powder and mix well. Allow it to cook for 2 minutes. If the sauce is too dry you can add a little more warm water
Chop the other half of the onion and the green pepper into cubes or lengthwise and add to the sauce. Cook for about 5 minutes or until it is slightly soft but not overcooked.
Add the chicken and bring to a boil, about 2 minutes. Season with more salt if required and add a pinch of sugar if the sauce is too acidic
Garnish with coriander and serve hot