Preheat the oven to 375°F and lightly grease a 9x9-inch baking dish.

Place the sliced zucchini in a colander, sprinkle with 1/2 teaspoon of the salt, and let sit for 15 minutes to draw out excess moisture.

Heat the olive oil in a large skillet over medium heat. Add the onion and cook for 3 to 4 minutes until softened.

Add the ground beef and cook until browned, breaking it up as it cooks. Drain excess grease if needed.

Stir in the garlic, marinara sauce, Italian seasoning, remaining 1/2 teaspoon salt, and black pepper. Simmer for 5 to 7 minutes until slightly thickened.

Pat the zucchini slices dry with paper towels or a clean kitchen towel.

Spread a thin layer of meat sauce in the bottom of the baking dish. Layer half of the zucchini slices over the sauce, then add half of the remaining meat sauce. Top with 1 cup mozzarella and 1/4 cup Parmesan.

Repeat with the remaining zucchini, remaining meat sauce, and the remaining mozzarella and Parmesan.

Bake uncovered for 30 to 35 minutes, until the cheese is melted, bubbling, and lightly browned around the edges.

Let the casserole rest for 10 minutes before slicing. Garnish with chopped parsley or basil and serve warm.