Place the tuna, water, salt, onion, curry leaves, ginger, and black peppercorns in a large heavy-bottomed pot and bring to a gentle boil. Reduce the heat and simmer until the tuna is completely cooked and the cooking liquid has developed a rich, savory flavor. Carefully remove the tuna pieces and set them aside for another dish, then strain the liquid to remove the aromatics and return the clear broth to the pot. Continue simmering the broth uncovered over low heat, stirring occasionally, until much of the water evaporates and the liquid gradually becomes thick, dark, and concentrated. This reduction can take several hours, so patience is essential. Once the mixture reaches a thick paste-like consistency, remove it from the heat and allow it to cool slightly before transferring it to a clean container.