Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.

Lay the sliced zucchini on paper towels in a single layer. Sprinkle with 1/2 teaspoon of the salt and let it sit for 10-15 minutes to draw out excess moisture. Pat dry with more paper towels. This is a crucial step to prevent a watery casserole.

Heat the olive oil in a large skillet over medium-high heat. Add the chopped onion and cook until softened, about 5 minutes.

Add the ground turkey to the skillet. Cook, breaking it up with a spoon, until no longer pink, about 6-8 minutes. Add the minced garlic and cook for another minute until fragrant.

Drain any excess fat from the skillet. Stir in the tomato sauce, drained diced tomatoes, oregano, basil, red pepper flakes (if using), the remaining 1/2 tsp salt, and the black pepper. Bring the mixture to a simmer and cook for 5 minutes for the flavors to meld.

Arrange half of the patted-dry zucchini slices in a single, even layer at the bottom of the prepared casserole dish.

Spoon the entire ground turkey mixture evenly over the zucchini layer.

Top with the remaining zucchini slices, arranging them in an even layer.

In a small bowl, toss together the shredded mozzarella and grated Parmesan cheese. Sprinkle the cheese mixture evenly over the top of the casserole.

Cover the casserole dish with aluminum foil and bake in the preheated oven for 25 minutes.

Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown.

Let the casserole rest for 10 minutes before serving. This allows it to set and makes it easier to slice. Garnish with fresh parsley, if desired.