For the Ricotta Filling: Mix together ricotta cheese, spinach, egg, Italian seasoning, and salt.
For the Lasagna Assembly: In a 13x9 baking dish, add 1/2 cup marinara sauce. Add 3 lasagna sheets on top of the marinara sauce.
Add sliced eggplant, followed by ricotta mixture, and mozzarella cheese.
Add another layer of marinara sauce, followed by 3 lasagna sheets, summer squash slices, eggplant, ricotta mixture, and mozzarella cheese. (Repeat 2 times for two more layers)
For the final layer add marinara sauce, 3 lasagna sheets, remainder of marinara sauce, and top with more mozzarella.
Bake for 45 minutes, until the cheese is melted and starting to turn slightly brown. Remove from the oven; let Lasagna cool for 20 minutes before cutting.
