Make the marinade by mixing all the marinade ingredients together in a bowl. Smear the mixture over the chicken pieces and leave in the fridge to marinate for 3-4 hours or overnight.
When ready to cook heat the oil in a large non stick frying pan over a medium heat. Fry the onions for 11 or so minutes, until they soften and turn light brown.
Tip in the marinated chicken pieces and fry, sealing the meat. Continue to stir fry for 5 minutes until the chicken is browned. Add the garam masala and stir well. Add the water or stock then season and bring to the boil.
Reduce the heat to low, letting the mixture simmer gently. Cover the pan and cook for 9-10 minutes, until the chicken is cooked through, stirring halfway through.
Add the green chillies and fresh coriander. Stir and serve warm with paratha or rice.