In a large bowl, add the fish, sprinkle salt and turmeric and rub on the pieces very well. Let rest for 10 minutes. In a small bowl, add the powdered spices and mix ith 2-3 tbsp of water. Keep ready.
Add the mustard oil to a heavy wok or kadai and let heat up till slightly smoky.
Carefully, slide the fish into the oil and shallow fry for 2 minutes on each side. Remove from oil and place in a plate.
In the remaining oil, crackle the bay leaf, nigella and green chillies for 30 seconds.
Add the onion paste next and on medium heat, brown for about 3 minutes. Once browned add the ginger and garlic and fry for a minute. Add the powdered spices paste next, a splash of water and fry the spices for 2-3 minutes untill you see that the oil starts seperating.
Next add the tomato, sugar and salt and fry for 2 minutes till the tomatoes have softened.
Add hot water to the masala and bring to a boil. Once boiling, reduce the stove and let simmer for 2 minutes. Add the fried fish to the gravy along with any oil thats leftover in the plate in which you kept the fish. Taste and adjust the salt once.
Let everything cook for 6-8 minutes untill the fish is cooked through. Remive from stove and mix in the lime juice. Let rest for atleast 20 minutes. Serve with steamed rice.