Wash the fish pieces thoroughly and pat dry. Sprinkle salt and turmeric powder and smear the fish pieces well. Keep aside for 10 minutes.
Grind grated coconut, green chillies and coriander leaves to a smooth paste with a little water.
Heat oil in a wide pan. Fry the marinated fish to a light golden brown and remove.
In the same pan, sizzle kala jeera.
Add chopped onions and fry till the onions turn slightly brown. Add garlic paste and fry for a minute.
Add the ground coriander paste. Sprinkle salt and sugar and combine all the ingredients well. Cook for 4-5 minutes till the oil starts to leave the sides and you get a thick gravy.
Gently slide in the fried fish into the cilantro sauce. Let it simmer for 4-5 minutes till it gets nicely coated with the cilantro gravy.
Drizzle mustard oil on top and remove the cilantro fish.
Serve Coriander Fish with steamed rice and a wedge of lime.