Rub the fish pieces with turmeric, lemon juice, and a pinch of salt, and set aside while you prepare the curry.

Blend the grated coconut, coriander seeds, and dried red chilies with a little water into a smooth, thick paste.

Heat the coconut oil in a clay pot, add the mustard seeds, and let them splutter, then add the curry leaves.

Add the chopped onion and sauté until soft, then stir in the ginger-garlic paste and chopped tomato and cook until the tomato breaks down.

Stir in the coconut-spice paste and cook, stirring frequently, until it darkens slightly and the raw smell fades.

Pour in the coconut milk and kokum along with the salt, and let the curry simmer gently until it thickens slightly.

Slide the marinated fish pieces into the curry and let them simmer gently, uncovered, until just cooked through.

Taste the curry and adjust the salt and tang, then remove from heat once the flavors come together.

Cover the pot and let the curry rest briefly so the fish finishes gently in the residual heat.