Heat the oven to 350ºF. Lightly grease an 8x8 inch baking dish.
Using a spiralizer, make zucchini noodles following manufacturer's directions.
Place the zoodles in a fine mesh sieve over a bowl. If there are a lot of long pieces, trim with clean scissors or a knife.
Toss the zoodles with kosher salt and the zoodles release excess moisture for about 20 minutes.
While the zucchini drains, whisk together the eggs, parsley, and Parmesan in a medium bowl.
Give the zoodles one last squeeze to get rid of any excess liquid.
In a medium skillet, melt the butter and add the zucchini noodles. Immediately turn off the heat and add the egg mixture. Quickly toss until evenly coated and creamy. Add the ricotta and cream cheese and toss until combined and there are no chunks. Transfer the mixture to the baking dish.
Heat a medium skillet over medium heat. Add the sausage and brown all over, about 5 to 8 minutes. Remove from the heat, and stir in the marinara sauce. Spread mixture over the spaghetti. Add the mozzarella cheese in an even layer over top.
Transfer the baking dish to the oven and bake 30 to 40 minutes, until the cheese has melted and is lightly browned on top. Cool 5 minutes before serving. Serve with additional parmesan, if desired.