Combine the ingredients to make fried fish (1 teaspoon ground turmeric, 1 tablespoon ground black pepper, 1 tablespoon red chily powder, 1 teaspoon vinegar and salt) together in a small bowl. Marinate fish pieces for half an hour.
To make the fried fish, heat oil in a pan, and shallow fry the pieces until golden brown. Set aside.
Heat ¾ cup oil in a deep pan. Splutter mustard seeds. Fry curry leaves. Add in ginger, garlic and green chilies. Fry again.
Now reduce the flame, and add ground turmeric, ground fenugreek, and red chili powder. Pour in ½ cup vinegar. Cover with a lid, and bring to a boil.
Add in the pieces pieces, and bring it to a boil again.
Notes
For more color and less heat, add Kashmiri chili powder. If you want more sauce, add boiled and cooled water.