Preheat your oven to 375°F (190°C). Salt the eggplant slices and let them sit for 15 minutes to release excess moisture. Pat dry with paper towels.
Brush eggplant slices with olive oil and arrange on baking sheets. Roast for 20-25 minutes until tender. This step prevents the lasagna from becoming watery.
While the eggplant cooks, sauté garlic in olive oil until fragrant. Add spinach and cook until wilted. Set aside to cool.
In a bowl, combine ricotta, 1 cup mozzarella, 1/4 cup Parmesan, eggs, basil, oregano, salt, and pepper. Fold in the cooled spinach mixture.
Spread 1/2 cup marinara sauce in a 9x13 baking dish. Layer eggplant slices to cover the bottom.
Spread half the ricotta-spinach mixture over the eggplant, followed by marinara sauce. Repeat layers.
Top with remaining eggplant, marinara, mozzarella, and Parmesan cheese.
Cover with foil and bake for 45 minutes. Uncover and bake for an additional 15 minutes until golden and bubbly.