Preheat oven to 400°F. Line a large baking sheet with parchment paper.
Slice zucchini into ¼-inch thick noodles using a mandoline. Stack the sliced zucchini onto the baking sheet, spraying each layer with avocado oil or olive oil and sprinkling with salt. Roast for 10 minutes, or until tender. Remove from heat and let cool slightly.
Meanwhile, prepare the ricotta mixture: In a medium bowl, mix together ricotta, egg, spinach, basil, garlic powder, and salt.
Spread ¾ cup of marinara sauce over the bottom of a 13x9 baking dish.
Assemble the manicotti: Spread ~1 tablespoon of the ricotta mixture onto the thickest end of the zucchini noodle, and then roll. Place the rolled zucchini noodle onto the marinara-topped baking dish, with the last end facing down. Continue with each zucchini noodle.
Spread the remaining marinara over the stuffed zucchini. Sprinkle with shredded mozzarella, and then top with shredded parmesan. Bake uncovered in the oven for 15-20 minutes, or until the cheese has melted.
Let cool for 5-10 minutes. Serve and enjoy!