Wash the shark well, season with little salt and set aside.
In your blender, add in onions, tomatoes, deseeded, soaked and drained Kashmiri chillies along with cumin seeds, peppercorns, turmeric powder, tamarind, garlic cloves, and ginger. Make a smooth paste, add water if required.
Heat oil in a heavy bottomed pan on medium heat. Add the ground paste, fry the paste until the rawness goes away, about 5-7 minutes.
Rinse the blender used for grinding the paste with about a cup of water, add to the pan. Adjust the quantity of water as per the desired thickness. Add a little salt, vinegar, sugar. Bring this to a boil.
When the curry starts bubbling, add the fish. Let the curry simmer on medium-low heat for another 5 minutes or until the fish is cooked through. Taste for seasonings, adjust with more salt, sugar or vinegar for tang.