Preheat the oven to 375°F (190°C).

Heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté for 3-4 minutes until soft and translucent. Add minced garlic and cook for 1 minute more, being careful not to brown it.

Add the ground turkey to the skillet. Cook until browned and no pink remains, breaking it up with a spatula as you go, about 5-7 minutes total. Let it sit undisturbed for a minute or two before stirring for better texture.

Stir in the diced tomatoes with juice, Italian seasoning, salt, and pepper. Let simmer for about 5 minutes to allow flavors to meld and liquid to reduce slightly.

In a 9x13 inch casserole dish, spread half the zucchini slices across the bottom in an even layer, overlapping them slightly like roof shingles.

Spoon half the turkey mixture over the zucchini, spreading evenly, then sprinkle with about one-third of the cheese.

Repeat layers with remaining zucchini, turkey mixture, and most of the remaining cheese. Top with breadcrumbs if using, then sprinkle with the last bit of cheese.

Bake uncovered for 30-35 minutes, until bubbly around the edges and golden brown on top.

Let the casserole rest for 10 minutes before serving to allow it to set up properly so slices hold together.