Marinate fish with salt and turmeric. Set aside for now.
Melt coconut oil in a sauté pan over medium heat, then saute the onions, pearl onions, 20 curry leaves, Serrano peppers. Sauté for 15 minutes, or until the onions turn brown.
Add the ginger and spices (black pepper, salt, turmeric). Stir-fry for 2 minutes. Remove half of this mixture and set aside for garnish.
Add the fish, 1 can of coconut milk, ¼ cup of water and the remaining curry leaves. Cook for 2 minutes, then cover the pan with a lid and cook for 5 minutes.
Add the remaining ½ can of coconut milk and the apple cider vinegar to the pan. Continue cooking until the fish is done.
Garnish with the onion mixture that you set aside earlier, then serve.