Place the fish in a bowl. Sprinkle the turmeric and salt over the fish and massage nicely. Set aside for 15 minutes.
Dry roast all the spices listed under "Spices" and grind them to a powder using your spice grinder. Mix the spices with red chilli & turmeric along with a couple tablespoons of water in a bowl and let stand.
In a cooking pot, add the oil. Once the oil is bit smoky add the onions next and let them brown. The onions have to be browned nicely for a good flavor. Once the onions are browned, add the ginger and garlic and saute for a minute or so till you smell the aroma.
Next add the curry leaves along with the spice paste, you can add a splash of water (about 3 tbsp) and cook everything together for a few minutes till you notice the oil separating. Dont saute a lot.
Add 1 cup warm water to the pot, add salt and let come to a slow simmer. Add the fish next to the pot and let cook on low heat for 7-8 minutes. Dont let boil. You can cover if you fish but really fish cooks fast.
Once you notice that the fish is white and firm, add the vinegar, tamarind and jaggery. Gently mix everything and let cook for another 2 mins.
Switch off the stove and cover the pot.Serve warm.