Preheat your oven to 375°F.
Slice the zucchini lengthwise into thin ribbons using a mandoline or a sharp knife. Lightly salt them and let drain on paper towels to remove excess moisture.
In a skillet over medium heat, add a splash of water or olive oil if desired, sauté the chopped onion and minced garlic until softened, about 2-3 minutes.
Add the lean ground turkey to the skillet. Season with Italian seasoning, salt, and pepper. Cook until browned, breaking it up into small crumbles, about 5-6 minutes.
Stir in the tomato sauce into the turkey mixture and let simmer for another 2 minutes.
In a small bowl, lightly beat the egg. Set aside the part-skim mozzarella cheese.
In a baking dish, layer a single layer of zucchini ribbons as the base. Spread half of the turkey and tomato sauce mixture over the zucchini, drizzle half of the beaten egg, and sprinkle a portion of mozzarella cheese.
Repeat with a second layer of zucchini, then the remaining turkey mixture, and finish with the remaining egg and mozzarella on top.
Cover the dish with foil and bake in the oven for 20 minutes. Remove the foil and bake an additional 5-7 minutes to allow the cheese to lightly brown.
Remove from the oven, let cool slightly, and serve warm.