Preheat oven to 375°F and grease a 9x13-inch baking dish.
Sprinkle zucchini slices with 1/2 teaspoon salt and let sit for 10 minutes, then pat dry with paper towels to remove excess moisture.
In a large skillet, brown the ground beef with onion and garlic over medium-high heat until meat is fully cooked, about 8 minutes, then drain excess fat.
Stir tomato sauce, Italian seasoning, remaining salt, and pepper into the beef mixture and simmer for 5 minutes.
Arrange half of the zucchini slices in the prepared baking dish, then spread half of the meat sauce over the zucchini.
Sprinkle 1 cup of mozzarella cheese over the meat layer, then repeat layers with remaining zucchini, meat sauce, and mozzarella.
Top with Parmesan cheese and bake uncovered for 35-45 minutes until bubbly and golden brown on top.
Let stand for 10 minutes before serving to allow the casserole to set.