Marinate fish pieces with salt, turmeric powder, red chilli powder and 1tsp mustard oil.Keep for 15-20mins.
Soak poppy seeds, yellow mustard seeds separately.
Make a paste of mustard seeds, poppy seeds, cumin seeds, coconut, garlic and green chilli.
Add more green chilli if you want more spice.
Take curd in a mixing bowl, add the prepared paste.Mix well.
Heat oil in a kadhai.Add panch phoron, dry red chilli, curry leaves and mix well.
When splutter, lower the gas, add the yoghurt mixture and stir continuously.
When oil separates add 1-1/2cup of water,salt and turmeric powder. Cook for few mins.
When boil comes add the fish pieces and potato pieces.Cook covered for 5mins.
Check the fish pieces, turn the pieces carefully and cook further for 4-5 mins more.
Finally add crushed garlic, slited green chilli and 1tbsp mustard oil(optional) on top and switch off.
Dahi machha besara is ready.Enjoy this creamy curry with steamed rice.
The authentic way is to use raw fish and tastes best too, but if you want you can shallow fry the fish pieces before adding to curry.
You can use rohu, katla or ilishi.
Note- Panch phoron is a mixture of Cumin, Brown Mustard, Fenugreek, Nigella and Fennel seeds.If you don't have yellow mustard seeds, then substitute it with brown mustard seeds.