In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic; sauté until the onion is translucent and fragrant, about 3-5 minutes.
Add the ground turkey to the skillet. Season with Italian seasoning, salt, and pepper. Cook until browned and cooked through, breaking it up with a spatula. This should take about 6-8 minutes.
In a greased 9x13 inch baking dish, spread half of the marinara sauce evenly at the bottom. Layer half of the sliced zucchini on top of the sauce. Spread the cooked turkey mixture over the zucchini. Pour the rest of the marinara sauce over the turkey. Top with the remaining zucchini slices.
Sprinkle the shredded mozzarella cheese evenly over the top. Finally, sprinkle the grated Parmesan cheese for an extra cheesy crust.
Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes. Then remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Allow the bake to cool for about 5 minutes. Garnish with fresh basil leaves if desired, then slice and serve warm.