Heat oil in a a kadai or pan and add the chopped onions.
Fry on medium heat for 12-13 mins till it is brown in colour.
Add the marinated chicken pieces, mix well and fry/bhunno on medium heat for 5 mins.
Continue to fry/bhunno on low heat for 5-6 mins till chicken is browned and oil separates.
Add the sliced tomatoes, mix well and fry it on medium heat for around 2 mins.
Now add 100 ml water, mix well and cover & cook for 5 mins on low heat till the tomatoes are soft and pulpy & oil separates.
Add the sliced spinach leaves and mix well.
Now add the slit green chillies, cumin powder and a pinch of salt.
Mix well, keep the heat on low and cover & cook for 15 mins stirring a few times, till chicken is tender and oil separates.