Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish.
- If using grated zucchini, place it in a clean towel and squeeze out excess moisture. If sliced, lightly sauté in a pan to reduce water content.
- In a large bowl, mix eggs, sour cream, garlic, salt, pepper, and Italian seasoning.
- Add in zucchini, onion, cheddar cheese, and Parmesan. Stir until fully combined.
- Pour the mixture into the prepared baking dish and spread evenly.
- In a small bowl, combine melted butter with bread crumbs or crushed crackers. Sprinkle evenly over the top of the casserole.
- Bake for 35–40 minutes, or until the top is golden brown and the center is set.
- Let rest for a few minutes before slicing and serving.