Apply salt, turmeric powder and lemon juice to the mackerel pieces and set aside to marinate for 10-15 minutes.
Meanwhile grind the triphal with ¼ cup water. Strain and discard the residue.
Grind together coconut, red chillies and triphal water to a fine paste. If using tamarind pulp, add that to the masala and grind till well blended.
Heat 2 tbsps coconut oil in a non-stick pan. Crush the garlic cloves slightly and add to the pan and sauté till golden.
Add the ground masala and sauté for 2-3 minutes. If using kokum petals, then do not add tamarind pulp while grinding the masala. Add the kokum petals now and mix well. Add 2 cups water and mix well. Let the mixture simmer for 3-4 minutes.
Add mackerel pieces and mix well. Adjust salt. Cover and cook till the fish is done.
Drizzle the remaining coconut oil on top, and cover immediately to trap the flavours.