In a small pan over medium-low heat, dry roast the coriander seeds, cumin seeds, peppercorns, and dried red chilies until fragrant, about 2 minutes. Transfer to a spice grinder along with the desiccated coconut, garlic, ginger, and 2 tablespoons of water. Grind into a smooth paste.

Heat 2 tablespoons of oil in a heavy-bottomed pot over medium-high heat. Add the lamb cubes in batches, searing until browned on all sides. Remove the lamb and set aside.

Add the remaining 1 tablespoon of oil to the same pot. Add the chopped onion and cook until golden brown at the edges, about 6 to 8 minutes. Add the ground spice paste and turmeric powder, stirring constantly for 3 to 4 minutes until the oil begins to separate from the paste.

Stir in the chopped tomatoes and salt. Cook, mashing occasionally with the back of a spoon, until the tomatoes break down into a thick sauce, about 5 minutes.

Return the browned lamb and any accumulated juices to the pot. Stir in the diluted tamarind paste and 1 1/2 cups of water. Bring to a boil, then reduce heat to low, cover, and simmer for 50 to 60 minutes, stirring occasionally, until the meat is fork-tender and reaches an internal temperature of 74°C / 165°F at the thickest piece.

Uncover the pot during the last 10 minutes of cooking if the sauce needs thickening. Adjust salt to taste and serve hot.