Slice or crush the pearl onions, ginger, and garlic. Break the green chilies into halves if desired. In a heavy bottom pan/wok/terracotta pot heat 2 tablespoons of coconut oil. When the oil is hot add the onion, garlic and ginger pieces to it along with the green chilies. Saute for a few minutes until the onions are soft. Add the mustard seeds , fenugreek seeds and the curry leaves to the oil. Cook for a minutes until the seeds splutter and the aromas fill the air.
Add the spice powders and salt to the pan along with a few drops of water. Stir fry for 30 seconds till the spices cook and water dries. Add the thin coconut milk and the kokum with its soaking water. Mix and let it come to a boil .
Slide the fish pieces in. Scoop the liquid with a ladle and pour over the top of the fish pieces. Cover and let cook for 5 to 10 minutes. Uncover, flip the fish slices if the slices are thick, and cook for another 5 to 10 minutes or till the pieces are cooked through.
Add the thick coconut milk and adjust salt. Let the curry come back to a slow boil over low heat. Take off the heat as soon as bubbles begin to appear on the sides. Cover and let rest for 20 minutes for the flavors to mature.
Serve immediately or refrigerate as soon as cooled to room temperature.
Pomfret, mackrel , cat fish and similar fish work well in this dish.
If using canned coconut milk, use watre mixed with i tbsp coconut cream as the thin coconut milk and use the cream for the thick coconut milk
If kokum is not available , soak a small piece of tamarind. Or remove the seeds and pulp from a medium size tomato and add it just before pouring the thin coconut milk .
For detailed instructions go to https://oventales.com/nadan-meen-curry-with-coconut-milk/ .