Marinate the fish pieces with half a teaspoon of turmeric and salt, then set aside for 15 minutes.
In a blender, combine the grated coconut, dried red chilies, coriander seeds, cumin seeds, garlic, ginger, and the soaked tamarind with its pulp.
Process into a smooth, fine paste using a little water if necessary.
Heat coconut oil in a deep pan or clay pot over medium heat.
Add the sliced onions and sauté until translucent.
Stir in the ground coconut paste and sauté for 2-3 minutes until the raw aroma dissipates.
Add water to reach the desired curry consistency and bring the mixture to a gentle boil.
Add the marinated fish and slit green chilies to the pan.
Simmer on low heat for 5 to 7 minutes until the fish is fully cooked and the flavors are integrated.
Adjust salt to taste and serve hot with steamed rice.