Soak the fish tamarind in 3 cups of warm water and set aside.
In a large saucepan or claypot [earthen pot], add the coconut oil and heat it up on medium high heat. Once warm, bring the flame down to low, then add the mustard seeds, and the fenugreek seeds.
Once these splatter, add the fresh ginger and garlic, the onions, and red chilies.
Add some salt, mix, and allow the ingredients to cook on medium-low heat until the onions become soft and lightly golden brown on the edges. This will take approximately 5-7 minutes.
Next, add the dry spices [red chili powder, ground Kashmiri chili, ground coriander, ground turmeric], black pepper, and curry leaves to the pan. Mix well and allow to cook until the spices separate from the oil and are aromatic. This will take 2-3 minutes.
Add some water to the curry and mix to achieve a smooth yet semi-thick paste, then add the soaking water from the tamarind and one piece of the dried fruit. Mix well once more and bring to a light simmer.
Once the curry simmers, reduce the flame to low again.
Lastly, add the pieces of fish and ensure they are covered with gravy, adding some more water if necessary. Let the curry simmer until the fish is cooked and the gravy thickens. This should take 10-15 minutes, depending on the thickness of the slices of fish.
Should you have time, allow the curry to rest and, before reheating and serving, remove the piece of tamarind.
Serve hot with basmati rice, parathas, chapati, kappa, or appam.