Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish with olive oil.

In a large skillet over medium heat, add olive oil and sauté the chopped onion and minced garlic until fragrant and softened, about 2 minutes.

Add the ground turkey to the skillet and cook until browned, breaking it up with a spoon for even cooking, about 5-6 minutes.

Stir in the drained diced tomatoes, dried Italian herbs, salt, and black pepper. Allow to simmer gently for 3-4 minutes.

Place half of the sliced zucchini in the prepared baking dish as the first layer.

Top with half of the turkey mixture followed by half of the shredded mozzarella cheese.

Repeat layers with remaining zucchini, turkey mixture, and cheese.

Bake the casserole uncovered in the preheated oven for 30-35 minutes, or until bubbly and golden brown.

Let it cool for 5 minutes before serving.