Using a mandoline, slice the zucchini squash lengthwise to make ¼ inch thick noodle shapes. Discard the first sliced cut of zucchini as it will be mainly the skin.
Lay the zucchini slices out onto a baking sheet lined with paper towels. Generously sprinkle with salt. Let sit at least 30 minutes to allow water to release from zucchini. Pat dry with paper towels.
In a large skillet, add Italian Sausage and diced onion. Cook until well browned.
Mix together the ricotta cheese, parmesan cheese, large egg, and parsley until well combined.
Preheat the oven to 375 degrees.
Begin layering the lasagna. Start with some ⅓ of the cooked meat and sauce on the bottom of the pan. Then add 1 layer of zucchini noodles. For me this took 3 slices and they were touching one another. You may need more or less depending on the size of your zucchini.
The next layer will be the ricotta mixture. Spoon half of the ricotta onto the zucchini slices and spread it around evenly. Then, spoon some of the vodka sauce over top of the ricotta, Just enough to have a thin layer overlaying it. Sprinkle with 1 cup of shredded mozzarella and another portion of the meat mixture.
Repeat the process for another layer. Then, for the final layer, place zucchini noodles on top and cover with sauce and the remaining cup of shredded mozzarella cheese.
Transfer baking dish to the oven and bake at 375 degrees for 50-60 minutes. The top of the zucchini will be browned and you will see the sauce bubbling.
Remove zucchini lasagna from the oven and let cool at least 15 minutes. I let mine sit for 30 minutes before slicing. It was still warm and it held together fairly well. Take note that because this is zucchini lasagna, it will not be as firm as one made with pasta. But, it is equally delicious and well worth making!