Heat 1 tbsp mustard oil in a non stick pan. Slice the ginger and garlic and add to the pan and sauté lightly. Break the green chillies and add along with the mustard seeds.
Add vinegar and switch off the heat. Transfer this mixture into a mixer jar and grind to a fine paste. Heat the remaining mustard oil in another non stick pan. Chop the onions finely.
Add the paanch phoron and onions to the oil and sauté lightly. Meanwhile sprinkle salt and ¼ tsp turmeric powder over the fish slices and apply evenly over all the slices.
Do not let the onions colour. Add the ground paste and mix. Add ½ cup water, salt and the remaining turmeric powder and mix. Add coconut milk and mix. Add the fish pieces and mix.
Cover the pan and cook till the fish is done. Serve hot with steamed rice.