1. Place a griddle on gas flame, place the banana leaf on it. press the leaf using a cotton cloth. this process will help you to fold the leaves easily without breaking corners.
2. In a bowl, add 500 gms betki fillet, juice of 1 lemon, salt to taste. now mix them well and keep aside for 15 to 20 minutes.
3. Now in a pot take 50 gms black mustard seeds, 50 gms yellow mustard seeds and add water to soak for 20 minutes.
4. After 20 minutes, strain it and add it in a mixer grinder along with 7-8 green chillies, salt to taste and little water. make a paste of the above ingredients.
5. Now take one big bowl, place the paste in it and add 3 tbsp curd, 1/2 tsp turmeric powder, 3 tbsp mustard oil. now add the marinated pieces of fish in it, mix them well and keep aside for 30 minutes.
6. After 30 minutes, place one fish piece on the banana leaf (medium sized), place some mustard paste on it, one green chilli at the top, fold it and tie with a thread.
7. In a frying pan add 2 tsp mustard oil and brush it on the surface. now place the fish pieces (covered with banana leaves) on it.
8. Cover it and fry it for 5 minutes on low to medium flame. open the cover, turn the pieces, cover it and let it fry. repeat the frying process for 10 to 15 minutes.
9. The bhetki paturi is ready. serve it with rice. enjoy.