Preheat your oven to 375°F (190°C) and lightly grease an 8×8-inch baking dish.

In a large skillet, heat 1 tablespoon of olive oil or butter over medium heat. Add ½ cup of chopped onions and 4 cups of sliced zucchini. Sauté for 5–7 minutes until tender.

Sprinkle in ½ teaspoon of garlic powder, ½ teaspoon of dried basil, salt, and black pepper to taste. Stir well and drain excess moisture if needed.

In a mixing bowl, beat 2 large eggs until smooth, then stir in ½ cup of sour cream, 1 cup of shredded cheddar, and ½ cup of grated Parmesan until well blended.

Fold the cooled zucchini and onion mixture into the cheese and egg blend, ensuring even distribution.

Pour the mixture into the prepared baking dish. In a small bowl, mix the remaining ½ cup of breadcrumbs with 2 tablespoons of melted butter and sprinkle it over the casserole.

Bake uncovered for 25–30 minutes until the top is golden brown and the center is set. Broil for an additional 2–3 minutes for extra crispiness if desired.

Remove from the oven and let it rest for 5–10 minutes before serving.