Preheat oven to 400 F. Line two baking sheets with parchment paper. Set aside.
Cut the eggplant lengthwise into ¾” slices. Brush them with oil and season with salt and pepper.
Roast the eggplant for 25 to 30 minutes or until golden, flipping the slices over halfway through the cooking time. When the eggplant has finished cooking lower the heat of the oven to 350 F.
In a large pan over medium heat, add oil and saute garlic until fragrant. Add the spinach and cook until slightly wilted. You might need to do this in batches. Remove spinach from pan once wilted and place on a paper towel lined plate to absorb excess moisture.
In a large mixing bowl, add the ricotta, ¼ cup of parmesan cheese, ½ cup mozzarella cheese, and egg. Mix until everything is well combined. Add in the spinach and continue to mix until combined.
In a 9 x 13 baking dish add enough marinara sauce to cover the bottom. Add the eggplant slices to the bottom of the dish and then add ½ of the ricotta mixture. Next add the marinara sauce. Repeat with another layer of eggplant slices, then the remaining ricotta mixture, and finally with sauce. On the final layer of sauce add the remaining ¼ cup of parmesan cheese and the remaining ½ cup of mozzarella cheese.
Cover the baking dish with foil, making sure to tent it in the middle so that the cheese does not touch the foil.
Bake for 20 minutes until the cheese has fully melted. Optional: remove the foil and broil for an additional 2 minutes to brown the cheese.
Let rest for 5 minutes and garnish with parsley.