Heat the coconut oil in a deep pan or traditional clay pot over medium heat.
Add the mustard seeds and fenugreek seeds. Let them splutter for about 30 seconds.
Stir in the sliced shallots, ginger paste, garlic paste, green chilies, and curry leaves. Sauté until the shallots turn soft and golden brown.
Lower the heat and add the Kashmiri red chili powder, coriander powder, turmeric powder, and salt. Cook for 2 minutes, stirring continuously until the raw smell of the spices disappears.
Pour in the water and add the soaked Kokum pieces along with their soaking liquid. Bring the mixture to a gentle boil.
Gently slide the fish chunks into the simmering sauce. Cover the pot and cook for 10 to 12 minutes until the fish is tender, flaky, and cooked through.
Reduce the heat to low. Carefully stir in the full-fat coconut milk and let it gently simmer for another 2 to 3 minutes. Do not let it boil vigorously to prevent the coconut milk from splitting.
Remove the pot from the heat. Let the curry rest for at least 15 minutes to allow the flavors to meld together before serving hot with steamed rice.