Gently flatten the eggplant using a fork and sprinkle a pinch of salt on it. Pour the beaten egg and coat the eggplant.
Combine the diced tomatoes and cucumber in a small bowl and mix well. Set aside.
Heat a large frying pan and add about 3-4 tablespoons of cooking oil. Fry the eggplant on medium heat, turning halfway once the other side is golden. Drain on a paper-towel lined plate and set aside.
Place the well-drained tuna in a large bowl and dredge carefully with cornstarch, making sure it’s evenly coated to prevent splatter while frying. Break the tuna pieces in big chunks for easy coating.
Heat a small frying pan on medium and add oil reaching about ¼ inch. Once hot enough, add the tuna and fry until golden and crispy. Drain on paper towels.
To assemble: Place the eggplant omelet on your serving platter. Sprinkle the diced tomatoes and cucumber then top with the crispy-fried tuna pieces. Drizzle with the fish sauce-lemon mixture or serve it on the side.