1. Cut off the heads, fins and tails of the Bombay ducks and soak in water for an hour. When the Bombay ducks softens, tear apart and remove the centre bone and shred the ducks with your fingers. Wash and drain.
2. Mix the onions, potatoes, Bombay ducks, chilli, turmeric, dhansakh powders, garlic, cumin, tamarind juice and jaggery in a clean bowl. Mix well. Add a pinch of salt.
3. Take a very large fish tava, pour one cup mustard oil on it and allow to heat well. Add the curry leaves and then the fish mixture. Lower the flame and mix well non-stop. After five minutes invert a vessel over the mixture and allow to cook on a very low flame in its own steam. Remove vessel, mix and cover again. See that the patia does not stick to the tava and burn.