Layer a rimmed sheet pan with paper towels and place one layer of zucchini planks on top. Season lightly with salt on both sides, then add another layer of paper towels. Repeat with another layer of zucchini, and add a final layer of paper towels on top. Add another sheet pan on top of the zucchini, and add a few cans or heavy pans to weigh it down. Let sit for 1 hour, then carefully blot dry, removing as much excess moisture as possible.
Preheat the oven to 400˚F. In a medium bowl, mix together ricotta, 1 ½ cups mozzarella, most of the parmesan cheese if using, parsley, basil, egg, garlic, Italian seasoning, red pepper flakes, salt and pepper.
Add about ½ cup of marinara sauce to a 9 by 13-inch baking dish and spread into an even layer. Add a layer of zucchini, taking care to cover the entire bottom of the zucchini. Add ¼ of the ricotta mixture and spread into an even layer. Top with about ¾ cup of marinara sauce. Add another layer of zucchini, ricotta and marinara. Repeat one more time, then add a fourth and final layer of just zucchini and ricotta. Sprinkle with remaining mozzarella and remaining parmesan, and bake for 45 minutes or until bubbling and golden brown on top.
Let cool for at least 30 minutes before cutting and serving so the lasagna sets up slightly. If there is a lot of excess water from the zucchini, you can spoon it off with a tablespoon and discard before slicing the lasagna.