Preheat the oven to 375°F.

In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. Stir in the minced garlic and cook for an additional 1 minute until fragrant.

Add the ground turkey to the skillet. Season with Italian seasoning, salt, and black pepper. Cook until the turkey is browned and cooked through, about 5-7 minutes.

In a large mixing bowl, combine the sliced zucchinis, cherry tomatoes, cooked turkey mixture, marinara sauce, and half of the mozzarella cheese. Stir until well combined.

Transfer the mixture into a greased 9x13-inch baking dish. Top with the remaining mozzarella cheese and the grated Parmesan cheese.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.

Let the casserole cool for 5 minutes before serving. Garnish with fresh basil if desired.