Clean fish well and remove scales etc and cut into small pieces.
Take a small frying pan and on a slow flame, dry roast the coriander powder till it turns a light shade of brown. Take extra care that you do not burn the powder.
Next add 1 tsp fenugreek powder to this and fry for a very short while, say less than a minute.
NOTE: Do not let the powder mix burn. This can spoil the taste of the curry completely.
Switch the flame off and add chilly powder to the rest of the powders in the fry pan. Mix all the powders well and keep aside.
In a blender, add the grated coconut. Add 6-7 shallots. Add all the fried powders. Add Turmeric powder, and required salt.
Do not add any water yet. Close the blender and powder the contents with a few spins.
Open the blender and add 1/4 tea cup of water and blend till the consistency reaches that of a fairly granular mixture.
Add little more water to it if necessary. Do not blend for very long. The masala should not become too smooth/watery.
Open the blender lid once again and add 5-6 curry leaves. Spin once or twice.
In a claypot/wok add the above masala from the blender.
Add kokum along with water used to soak it. Let it boil in medium flame.
Add the fish and 3 green chillies. Add salt as required.
Add 1/2 tea cup water. Using cloth/oven mittens, take the clay pot with both hands and rotate the clay pot gently thereby mixing the fish and masalas well. Using a spoon might break the fish pieces. Allow it to boil.
Cover and cook till done. Rotate the clay pot twice or thrice in between cooking as done in the above step, to avoid the curry from burning.
Transfer to a serving dish and serve along with rice.
Note: When making Mackarel/Vaala fish you can also add Tomato and Drumsticks (cut into pieces) in the step where you add the fish to the claypot. Raw mango pieces can also be added instead of Tomato.