Cut off the head of the fish and clean the insides.
Wash thoroughly under running water.
Cut the fish horizontally into 1-inch-thick (2 1/2-cm-thick) slices and put in a bowl.
Add the lemon juice, 1/2 teaspoon of the turmeric, and the salt, and stir well.
Cover the bowl and put it in the refrigerator for 15 to 20 minutes to marinate.
Place a small nonstick saute pan over medium heat.
Add the cumin and coriander, and dry-roast for 2 minutes or until fragrant.
Let cool, then transfer to a food processor with the dried chiles, the remaining 1/2 teaspoon turmeric, the coconut, half of the onions, the peppercorns, tamarind pulp, and 1 cup (200 ml) water, and process to a smooth paste.
Place a medium nonstick saucepan over medium heat and add the oil.
When small bubbles appear at the bottom of the pan, add the remaining onion and saute for 3 to 4 minutes or until lightly browned.
Add the tomato and green chiles and saute for 2 to 3 minutes or until the oil comes to the top.
Add the coconut paste and cook over high heat for 1 minute.
Add 1 1/2 cups (300 ml) water and bring to a boil.
Lower the heat to medium, add the marinated fish, and cook for 5 to 6 minutes or until the fish is cooked through.