Sauté the Onion and Garlic: In a large saucepan, heat the olive oil over medium heat. Incorporate the chopped onion and sauté for 5 to 7 minutes, or until it becomes tender. Then, add the garlic and continue cooking for an additional 1 to 2 minutes, until it releases its fragrant aroma.
Simmer the Sauce: Add the tomato passata, salt, and pepper. Bring to a simmer, then reduce heat and cook uncovered for 20–25 minutes, stirring occasionally, until the sauce thickens. Adjust salt and pepper to taste.
Preheat the Oven: Preheat the oven to 400°F (200°C). Cover a baking sheet with parchment paper to prevent sticking and ensure easy cleanup.
Prepare the Eggplant: Cut the eggplants into ¼-inch slices. Arrange the slices on the prepared baking sheet, then brush both sides with olive oil and sprinkle with salt.
Roast the Eggplant: Roast the eggplant in the preheated oven for 20–25 minutes, flipping halfway through, until tender and lightly browned. Remove from the oven and set aside.
Layer the Ingredients: In a baking dish (about 9x13 inches), spread a thin layer of tomato sauce on the bottom. Arrange a layer of roasted eggplant slices on top. Evenly distribute a layer of Parmesan, mozzarella, and fresh basil leaves over the top.
Repeat Layers: Continue layering tomato sauce, eggplant, cheeses, and basil until all ingredients are used, finishing with a top layer of sauce and a final sprinkle of cheese.
Bake: Reduce the oven temperature to 375°F (190°C). Cover the dish with aluminum foil and bake for 20 minutes. Then, remove the foil and continue baking for an additional 15 to 20 minutes, or until the cheese is bubbly and has turned a lovely golden brown.
Serve: Let the Eggplant Parmigiana cool for 10 minutes before serving. Garnish with extra basil leaves, if desired, and enjoy!