Heat oil in a kadhai or pan, add the mustard seeds and when it splutters, add the pieces of green chillies. Give a stir and add the chopped onions. Fry on medium heat for 8 mins till slightly brown.
Now add the chopped tomato, cook on medium heat for 2 mins and add 1/4 tsp salt. Continue to cook on medium heat for another 5 mins till the tomatoes are soft & oil separates.
Add the marinated chicken, mix well and bhunno/fry on medium heat for 5 mins.
Now add the Kashmiri Chilli Powder, mix well & fry on medium heat for 3 mins. Reduce the heat to low and continue to fry for another 2 mins till oil separates.
Add 150 ml water, mix and cover & cook on low heat for 15 mins.
Remove lid, mix in the ginger juliennes and simmer for around 2 mins till oil separates.
Serve with rice or roti