Sprinkle the turmeric and about a ¼ tsp of salt over the fish, mix well and set aside for 10 mins while you grind the spice paste.
Drain the soaked chillies (if using whole chillies and reserve some of the water to add to the curry later) and grind to a fine paste along with all the other ingredients for the masala including the vinegar.
Heat the vegetable oil in a deep pan, add the mustard seeds and curry leaves and allow the seeds to crackle.
Now add the sliced shallots and saute on medium heat till they are translucent.
Stir in the masala paste and cook on low-medium flame for 3-5 mins, adding a little water if necessary to prevent the spices from burning.
Once you notice the oil separating on the sides, add a little more water (not too much, the gravy must be thick), season and slide in the fish pieces.
Bring to a boil, lower the heat and simmer for 10-15 minutes till the fish is cooked but not falling apart.
Turn off the heat, pour the coconut oil over, stir and serve.