Use the same oil for making the masala. Transfer ½ cup oil to another pan. Add more fresh oil if needed.
Heat the oil over medium high heat. When the oil is hot reduce heat to medium.
Add mustard seed and let it crackle, then add 1/2 teaspoon fenugreek seeds and 2 whole red chilies.
When the spluttering stops, add chopped ginger, garlic, green chilies, curry leaves, 1 tbsp salt
Saute for 10 minutes over medium heat until the garlic and ginger start to get light brown. It does not need to be brown.
Reduce heat to low and then wait for 30 seconds before adding dry spices. Make sure the heat is very low.
Add 1/2 tsp turmeric, 4 tablespoons Kashmiri chili powder, 2 tablespoon chili powder, 1/2 tsp black pepper. Make sure to adjust the amount of chili powder to your spice preference.
Stir for 10 - 20 seconds. Make sure not to burn the spices.
Add 1 1/2 cup vinegar and 1 tablespoon salt. Let it boil over medium heat.
Reduce heat and add fried fish and mix it very well. Check for salt and consistency, adjust as needed. If you like more gravy you can add 1/2 cup hot water and boil it.
Turn off heat and let it cool completely.
Store in an airtight container and you can keep it in the fridge for up to 6 months. It is best to serve it after a week as it takes time for the flavours to settle into fish pieces.