Preheat the oven to 200°C (400°F).
Bring a large pot of lightly salted water to a boil and cook the spaghetti until al dente, about 3–4 minutes before the package time; drain and transfer to a large bowl.
Stir two-thirds of the tomato sauce and the chopped basil into the drained spaghetti until evenly coated.
Heat a generous amount of oil in a frying pan over medium heat. Fry the eggplant slices in batches without overcrowding until golden but not crispy, about 4–6 minutes per batch.
Drain the fried eggplant slices on paper towels and season lightly with salt.
Spoon a small amount of tomato sauce over the base of a baking dish. Place a forkful of sauced spaghetti on one eggplant slice, sprinkle with some cubed mozzarella, fold or cover with another slice as a 'blanket', and transfer the sandwich into the baking dish. Repeat with remaining slices to make 6 sandwiches.
Pour the remaining tomato sauce over the assembled eggplant sandwiches, then sprinkle with the grated Parmesan and any remaining mozzarella.
Bake in the preheated oven for about 10 minutes, until the cheese is melted and the top is lightly golden.
Season with freshly ground black pepper, garnish with basil leaves, and serve immediately.
