Heat the oil in a heavy-bottomed pan over medium heat and gently sauté the onion, garlic, ginger, green chilies, and curry leaves until fragrant and the onion begins to soften. Add the turmeric, cumin, coriander, black pepper, and chili powder, stirring for about 30 seconds to release their aromas. Add the chopped tomato and cook until it softens into the spice mixture. Pour in the coconut milk and water, season with salt, and bring the curry to a gentle simmer. Add the tuna pieces and cook gently for about 8–10 minutes, or until the fish is just cooked through and tender. Avoid vigorous boiling, as it can make the tuna dry and break apart. Finish with fresh lime juice and garnish with chopped coriander before serving hot with steamed rice or warm roshi.