Take 3 pieces of fish (large King Mackerel), rub ½ tsp of salt and keep them aside for at least 45 minutes to 1 hour to marinate. (Pro tip: Take 5 to 6 pieces if the fish is small).
Now, in a bowl, take 1 piece of tamarind (small lemon size) and put 4 tbsp of hot water in it. Let it soak nicely and then extract the pulp.
Then, in a mixer, take half a piece of coconut (grated), 12 pieces of Kashmiri red chilies (whole without stalk), 6 cloves of garlic (deskinned), 1 piece of ginger (½ inch size, peeled and diced roughly), 1 tsp of cumin seeds (jeera), 3 pieces of black peppercorns, 1 tbsp of coriander seeds (dhania), ½ tsp turmeric powder (haldi), and ¾ cup of water. Also, add the tamarind pulp you have extracted to it.
Grind them all to make a smooth paste and transfer it to a bowl. Keep it aside.
Now, take a frying pan. Do not turn on the gas stove. First, put ½ cup of chopped onion and salt to taste. Mix the salt and crush the onions further with your fingers.
Then, add 3 pieces of green chilies (slit in the middle) in the pan and pour 1 tbsp of cooking oil as well. Now, switch on the gas stove. Stir continuously and fry them on a medium flame for about a couple of minutes or so.
Now, when the onions turn slightly brown in color, add the masala paste you made before. Stir well to mix the masala paste with the onions and fry it for a minute or two on a low flame.
Now, pour two large mugs of water to the masala. Mix it nicely with the spices.
Then, add 5 pieces of dry kokum to it. Mix them nicely and bring the curry to a boil.
When it does, add the marinated fish pieces to it. Cover the pan and let it cook for 5 minutes on a medium flame.
Now, remove the cover and cook the curry on a high flame for about two minutes or so to thicken the gravy. Finally, your homemade tasty and spicy Goan fish curry is ready. Transfer it to a serving tray and enjoy it hot with steamed rice.