Grind mustard seeds, salt, turmeric powder, 2 green chillies with a little water.
Transfer the paste into a bowl, add fish slices and mix well.
Add 2 tbsps mustard oil and mix.
Heat the remaining oil in a non stick pan till it smokes and take it off heat.
Heat it again, add the fish pieces and sauté for 2-3 minutes.
Turn over and sauté the other side too.
Slit the remaining 2 green chillies and add with 1 cup water and mix.
Add red chilli powder and mix.
Add 1 cup water to the mustard paste remaining in the bowl and add to the pan.
Cover and cook till fish is just done.Serve hot with boiled rice.