Heat the oil in a large, deep frying pan (or wok) and add the garlic, green chillies and ginger. Mix them for a minute in hot oil then add the chopped onions. Stir and leave to cook over a medium heat for 8 to 10 minutes.
Once the onions have turned a golden colour, add the turmeric, chilli, cumin, coriander and garam masala powders. Stir and cook for about a minute.
Add the chopped tomatoes with 100ml water and a good pinch of salt. Stir together, cover the pan and allow it to come to a boil. Stir occasionally until the masala becomes nice and thick.
Add the chicken to the pan and coat the pieces in the masala so it’s all nicely covered. Cook over a medium heat, stirring occasionally for 15 to 20 minutes, or until the chicken is tender and combined with the thick masala sauce. (Leave lid off for last 5 minutes to reduce if it needs to be thickened.).
Stir in the lemon juice to add a little zing and garnish with coriander (if using) to serve, with basmati rice and/or roti/naan.