Smear the fish with a paste of salt, turmeric and pepper and fry till cooked.
Cook the tomato in a cup of water and take the puree through a sieve.
Grind the skin of the chillies, ginger and garlic.
In a flat vessel or frying pan, fry a half cup of onions.
Add the masala, fry well and then saute the onion pieces with it.
Pour the vinegar, puree, salt, soyabean sauce and 1 cup of water.
When it boils well, put the fish.
Season with sugar and allow the gravy to thicken.
Garnish with onions cut into roundels.