Preheat the oven to 400°F. Place one rack in the middle of the oven and one rack in the upper third. Line two baking sheets with cooling racks. Place the eggplant slices on the racks and season generously with salt. Let them sit 15 minutes, then dab excess moisture with a paper towel. Flip the eggplant slices and repeat.
In a large skillet, heat the oil over medium heat. Add the onion and cook until softened and translucent, 5 minutes. Add the garlic and Italian seasoning, cooking until fragrant, 1 minute more. Season to taste with salt and pepper. Add the marinara sauce and ¼ cup fresh basil, stirring to combine. Simmer until warmed through, about 3 minutes. Remove from the heat.
In a medium bowl, whisk the egg, then mix in the ricotta, ¾ cup Parmesan, remaining ¼ cup basil, and freshly ground black pepper.
In a 13x9 baking dish, pour ⅓ of the marinara on the bottom of the pan. Top with ⅓ of the eggplant slices, then ⅓ of the ricotta mixture, and 1 cup shredded mozzarella cheese. Repeat this sequence an additional 2 times, finishing off the final sprinkling of mozzarella with the remaining ¼ cup Parmesan and more black pepper. Lightly grease a sheet of foil and place it on top of the lasagna, greased side down. Bake on the middle rack until the eggplant is softened and the cheese is melted, 30 minutes. Remove the foil.
Preheat the broiler to high and transfer the lasagna to the top rack. Broil until the cheese is slightly brown and bubbling, 2–3 minutes. Remove and let cool at least 10 minutes before slicing. Garnish with more fresh basil and serve immediately.