Cut up the chicken into medium sized (3 inch) pieces, apply salt and set aside.
Put 2 tablespoons of oil in a pot. When heated, put in the sliced onions and green chillies and fry till the onions turn light brown. Put in the ginger paste, garlic paste, pepper, tumeric and cumin powder and fry.
Add the chicken and mix well. Pour in the water, cover and cook. When the chicken is tender put in salt, vinegar and tamarind. Add the coconut milk and bring to a boil. It's ready...
You could make it without the coconut milk and it would still be good.