This is not a typical kabab dish. Here, the Ilish/hilsa fish is cooked as a whole fish after being marinated with various flavors. It is a great item for grand occasions, and is a popular dish amongst Bengalis.
Cut the head of the fish in such a way so that it completely does not come off. It should be such that you are able to clean the belly of the fish and its head. Then clean out the scales. Wash and drain the whole fish.
Place the fish in a big dish. Add vinegar, salt and turmeric powder. Cover the ingredients on both sides of the fish. With a fork, make slits on the surface of the fish, so that the ingredients go well inside.
In a separate bowl, mix yoghurt, all the pasted ingredients, powdered ingredients and tomato sauce. Then pour this mixture on the fish and reverse the sides after a few minutes to allow even absorbing of the ingredients. Marinate for 2 hours.
In a flat pan, heat oil and ghee together. Fry the onions until crispy and golden brown. Transfer the fried onions on to a plate. Then add the fish on the pan and cook and fry on low heat. Every now and then reverse the sides of the fish for even frying.
When the fish has been fried well and the oil has started to simmer on top, the fish is ready. Transfer the onions back and mix it along with fish sides.
When serving, cut the fish.