1. Clean, wash and cut the fish into medium sized pieces.
2. Apply some salt to the fish pieces and keep aside for 15-20 minutes.
3. Grind the mustard seeds and green chillies to a fine paste, adding very little water while grinding it.
4. Add mustard oil, yoghurt and a dash of salt to the mustard and green chilli paste. Mix well.
5. Coat the fish pieces with the mustard paste.
6. Place the fish in a microwave and cook for 5-7 minutes.
7. Remove and check whether done or keep for a couple of minutes more.
8. Serve hot with steamed rice.

P.S: Do not add any water while cooking the fish. You can also steam the fish on gas.
To Steam the Fish on Gas:

1. Place the fish in a closed container.
2. In a heavy bottomed vessel, put about two glasses of water.
3. Keep a medium sized bowl with little water in the centre of the vessel.
4. Rest the container with the fish on the bowl.
5. Make sure that the container is above the water level in the vessel.
6. Cover the vessel with a lid and cook for about 10 minutes till the fish is done.