Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with cooking spray.
Season both sides of zucchini slices with salt and pepper.
Spray a non-stick skillet with cooking spray and place over medium-high heat; cook and stir zucchini slices until lightly browned, 2 to 3 minutes per side.
Spread 1/2 cup marinara sauce in the prepared baking dish. Layer zucchini slices, overlapping slightly, on top of marinara layer. Spread 1/2 cup marinara sauce over zucchini layer; top with 1/4 cup Parmesan cheese and 1/4 to 1/2 cup mozzarella cheese.
Continue layering zucchini, 1/2 cup marinara, 1/4 cup Parmesan cheese, and remaining mozzarella cheese.
Bake in the preheated oven until cheese is melted, about 10 minutes.
