Marinate cleaned fish pieces with 1/2 tsp turmeric powder, 1 tsp chilli powder and salt for half an hour.
Heat vegetable oil in a deep and wide pan. Deep fry the marinated fish pieces till it becomes crispy and dark brown in color. (refer notes)
Strain the oil used for frying and keep it aside.
Add sesame oil to the same pan and crackle mustard and fenugreek seeds. Add sliced ginger, garlic, and green chilli. Fry till golden brown and transfer to a plate.
Make a thick paste by adding vinegar to turmeric powder (1 tsp) and chilli powder (1 tbsp).
Add ginger – garlic paste to the same oil (sesame) and fry for 2 mins or till the raw smell goes. Add vinegar spice paste to this. Fry for 2-3 mins or till it gets roasted. Add 1 tsp sugar to this. Mix well.
Add 1 cup boiled and cooled water to this and bring it to boil. Add fried fish pieces along with fried ginger, garlic, and green chilli. Let it boil.
Reduce the flame to the lowest and cook for another 5 mins. Chech for salt and vinegar. If more vinegar is required, boil it before adding to the pickle. Remove from fire.
As the pickle cools add the strained oil (oil used for frying fish) little by little. Once the pickle cools down completely, pour the remaining oil (oil used for frying fish) on top. Press down the fish pieces in the bottle, so that oil comes to the top.