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