Preheat the oven to 350°F (175°C).
Prepare the zucchini by slicing it into thin rounds.
In a skillet, sauté the diced onion until translucent, about 5 minutes. Add minced garlic and cook for another minute.
In a large bowl, combine sautéed onions, garlic, zucchini slices, sour cream, cheddar cheese, mozzarella cheese, Italian seasoning, salt, and pepper.
Transfer the mixture to a greased 9x13-inch baking dish.
If desired, top with breadcrumbs for added crunch.
Bake for 30-35 minutes or until the casserole is bubbling and the top is golden brown.
Let the casserole sit for 10 minutes before serving.