Heat oil in a pan & splutter mustard seeds. Add chopped onion & stir until it becomes brown on low fire. Mix all the powders together with a little water & make a paste. Add this to the Onion and stir well. After 5 minutes add tomato puree & stir well. Add one cup thin coconut milk. When it starts boiling add chicken pieces. Add salt, ginger, garlic, green chillies & curry leaves. Cook for 10-15 minutes till it is tender. When the gravy becomes thick add thick coconut milk. Let it boil. Optional - add 1 teaspoon vinegar while taking it off from the stove.