Place the tuna pieces in a large pot with the water, salt, onion, garlic, ginger, black peppercorns, and curry leaves. Bring the mixture to a gentle boil, then reduce the heat and simmer for about 20–25 minutes, skimming any foam from the surface as needed. Continue cooking until the tuna is tender and the broth becomes clean, savory, and fragrant. Taste and adjust the salt before removing it from the heat. Serve the hot garudhiya with steamed rice, sliced chili, onion, and plenty of fresh lime wedges on the side.