Begin by preheating your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until soft, about 4 minutes.
Add the ground turkey to the skillet and cook, breaking it apart with a spatula, until fully browned (around 6-8 minutes). Incorporate the minced garlic and oregano, and cook for an additional minute until fragrant.
Stir in the marinara sauce until everything is well combined and heated through.
In a greased 9x13-inch baking dish, arrange half of the zucchini slices on the bottom. Spoon half of the turkey mixture over the zucchini, then layer with the remaining zucchini and finish with the rest of the turkey mixture.
Sprinkle the mozzarella and parmesan cheese evenly on top. Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and continue baking for another 10-15 minutes or until the cheese is bubbly and golden.
Let the casserole rest for 5 minutes before serving. Garnish with chopped parsley for a fresh touch.