Wash the kokum and leave it to soak in 50ml warm water and cut it into 12 pieces. Rinse the fish, pat dry and rub in ½ teaspoon turmeric and ½ teaspoon salt. Set aside to marinate.
Dry-roast the fenugreek seeds for a few seconds in a small frying pan till fragrant, then grind. Be careful not to over-roast the seeds as this will make them bitter.
Mix the fenugreek, chili paste and half a teaspoon of turmeric together with 50ml of water, forming a paste.
Heat the oil in a deep heavy-based frying pan. Add the mustard seeds; when they pop after a few seconds, add the curry leaves, onion and garlic and fry until the onion is translucent (about 3-4 minutes), then add the remaining salt.
Now add the fenugreek paste and fry, stirring, for a further 3-4 minutes. Add the remaining water, bring to the boil, reduce the heat and simmer for 20 minutes.
Add the soaked kokum plus its soaking water. Simmer for a further 10 minutes, until the mixture thickens a little.
Add the monkfish and simmer gently for 10 minutes. Serve with plain boiled rice.