1. Wash fish pieces and marinate with salt and turmeric powder and set aside for ten to fifteen minutes. Grind mustard seeds with green chillies to a fine paste using water as required.
2. Heat oil in a pan and lightly fry the marinated fish pieces on both sides till lightly browned.
3. Drain and set aside on a plate. In the same oil fry the potato fingers till lightly browned. Drain and set aside.
4. In the remaining oil add onion seeds and sauté till fragrant. Add the ground paste and sauté for a minute. Add a cup of water and bring it to a boil. Add turmeric powder and salt. Add potato fingers and simmer till they are done. Add fish, cover and simmer for two minutes.
5. Serve hot with boiled rice.