Keep the mustard seeds on a strainer, wash under running water. Now grind them with green chillies and 1/4 cup water using your food processor until a smooth paste form (read note). 

Peel and cut Potato into halves lengthwise, then cut each half lengthwise into 3 pieces. Wash and keep aside.

Rub salt and a 1/2 tsp of turmeric powder on the freshly washed pieces of fish. Heat 1/2 cup oil in a deep frying pan/kadai and shallow fry the fishes over medium heat, keep aside. 

Now fry the potatoes, let them turn golden brown in color, remove & keep aside.

In the remaining oil (3-4 tbsp oil should be there in the kadai, otherwise add some more oil) temper with onion seeds. Once it splutters add onions and sauté them until light brown in color, add in 1 cup of water to it.

Next to add mustard paste, 3/4 tsp turmeric powder, kashmiri mirch powder, and salt. Mix well and give this a quick boil over high heat. 

Once it boils lower the heat, add the fried fishes and potatoes. Cover and cook over low heat until potatoes are done (12-15 minutes approx.). Check salt and turn the heat off. Garnish with freshly chopped coriander leaves.

Serve hot with steamed rice.