Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and minced garlic, sauté until translucent.
Add the ground turkey or chicken to the skillet, cooking until browned and fully cooked, breaking it up as it cooks.
Stir in the bell pepper and mushrooms, cooking for about 5 minutes until softened.
Season the meat mixture with Italian seasoning, salt, and black pepper, and add the marinara sauce. Stir everything together and let it simmer for another 2-3 minutes.
In the bottom of the prepared baking dish, layer the sliced zucchini evenly.
Pour half of the meat and marinara sauce mixture over the zucchini layer, spreading it evenly.
Sprinkle 1 cup of the shredded mozzarella cheese over the meat layer.
Add another layer of zucchini slices, followed by the remaining meat sauce mixture. Top with the remaining zucchini slices and finish with the remaining mozzarella and Parmesan cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbling and lightly golden.
Once done, let the casserole sit for about 5 minutes before serving. Garnish with fresh basil leaves if desired.