Place the tuna slices on a clean surface and gently flatten them if necessary. Combine half of the onion, garlic, ginger, green chili, grated coconut, turmeric, cumin, coriander, black pepper, chili powder, and a little salt to create a fragrant filling. Spread a small amount of the mixture over each tuna slice, then carefully roll the slices tightly and secure them with toothpicks or kitchen twine. Heat the coconut oil in a wide pan and gently sauté the remaining onion, garlic, ginger, chilies, and curry leaves until fragrant. Add the remaining spices and stir briefly, then pour in the coconut milk and water and bring the sauce to a gentle simmer. Carefully place the tuna rolls into the curry, cover the pan, and simmer gently for about 15–20 minutes, turning the rolls carefully once or twice so they cook evenly. Finish with fresh lime juice, remove the toothpicks or twine, and serve the rolls hot with steamed rice or warm roshi.