Take the finely sliced onions, tomato and green chillies in a bowl.
Add a little salt and a pinch each of turmeric powder and red chilli powder. Add the grated ginger and 3 tbsp oil. Using your hand, mix well, softly mashing the onions, tomato and chillies as you mix.
Transfer to a pan, add the potato sticks and cook for 10-12 minutes over a low flame.
Now add the fish, give it a gentle mix. Be delicate, you do not want to break the fish.
Cover and cook for 2-3 minutes over low flame. Give it a gentle stir.
Add 1/4 cup warm water, let it cook for another 5-7 minutes.
Adjust seasonings. Finish off with a drizzle of mustard oil and a handful of chopped coriander leaves.