Lay the zucchini slices in a single layer on paper towels and sprinkle evenly with 1/2 teaspoon of the salt. Let sit for 10 to 15 minutes to draw out moisture, then pat both sides very dry and set aside.

Preheat the oven to 375°F and lightly grease an 8 by 8 inch or similar baking dish, unless using an oven safe skillet to bake in.

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

Add the minced garlic and cook for 30 seconds, stirring, until fragrant.

Add the ground beef to the skillet, break it up with a spoon, and cook for 5 to 7 minutes until fully browned with no pink remaining. If there is excess grease, drain it off.

Stir in the tomato sauce, Italian seasoning, remaining 1/2 teaspoon salt, and black pepper. Simmer for 5 to 7 minutes until slightly thickened, then remove from heat.

Spread a thin layer of the beef mixture over the bottom of the prepared baking dish or oven safe skillet. Arrange a snug layer of zucchini slices over the top.

Spoon more of the beef mixture over the zucchini and spread to the edges. Repeat layering zucchini and beef, finishing with a layer of the beef mixture on top.

Sprinkle the shredded mozzarella evenly over the surface of the casserole.

Bake for 25 to 30 minutes until the cheese is melted, bubbly, and lightly golden around the edges.

Let the casserole rest for 5 minutes before slicing and serving so the layers can set.