Preheat your oven to 400°F (200°C). Slice the eggplants into uniform 1/4-inch rounds, then lay them out on a baking sheet lined with parchment paper.
Brush each slice lightly with olive oil using a pastry brush, then sprinkle with salt and black pepper to taste. Place the baking sheet in the oven and roast for about 20 minutes, until the slices are tender and golden around the edges.
While the eggplant roasts, heat a small skillet over medium heat and sauté the minced garlic in a tablespoon of olive oil just until fragrant, about 30 seconds. Add the tomato sauce and simmer gently for 5 minutes to deepen the flavor.
Once the roasted eggplant slices have cooled slightly, spread a thin layer of tomato sauce on a baking dish or ovenproof dish. Layer the roasted eggplant slices over the sauce, then spoon more sauce over each layer.
Sprinkle shredded mozzarella and grated Parmesan cheese evenly over the layered eggplant slices. Repeat the layering process if the dish size allows, finishing with a generous cheese layer on top.
Place the assembled dish into the oven and bake for about 20 minutes, until the cheese is bubbly and golden brown. Keep an eye on it to prevent over-browning.
Remove the dish from the oven and let it rest for 5 minutes. Drizzle with balsamic vinegar if desired for a touch of sweetness and acidity, then serve warm.
