Preheat oven to 375°F (190°C) and grease a 9x13 inch baking dish.
In a large skillet over medium-high heat, cook the ground beef and diced onion until the meat is fully browned.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
Drain any excess grease from the pan.
Stir in the tomato sauce, dried oregano, salt, and black pepper; simmer for 5 minutes.
Arrange half of the zucchini slices in the bottom of the prepared baking dish.
Spread half of the ground beef mixture over the zucchini layer and sprinkle with 1 cup of mozzarella cheese.
Repeat with the remaining zucchini slices, remaining beef mixture, and top with the remaining 1 cup of mozzarella cheese.
Bake in the preheated oven for 30 to 35 minutes or until the zucchini is tender and the cheese is golden and bubbly.
Let the casserole rest for 5 to 10 minutes before serving to allow liquids to set.