Toss fish with half of the turmeric and salt. Leave to marinate for 15 minutes.
Soak kokum in ½ cup of warm water.
In a dry skillet, toast fenugreek and chiles until fragrant. Using a spice/coffee grinder, grind to a fine powder
In a skillet, heat oil. Add mustard seed. After a few seconds, add curry leaves, onion and garlic. Saute until onions are translucent.
Add fenugreek chile powder. Add kokum with its water plus 1 cup water and bring to a boil. Reduce to a simmer and cook for 20 minutes.
Add fish and simmer until cooked through, about 10 minutes.
Taste and adjust salt, if necessary.