Bring a large pot of salted water to a boil. Add zucchini slices and parboil for 3 minutes. Drain well in a colander and let cool for 5 minutes.

Preheat oven to 350°F. Grease a 2-quart baking dish with butter or cooking spray.

In a large mixing bowl, combine parboiled zucchini, mayonnaise, 1 1/2 cups cheddar cheese, beaten eggs, garlic powder, pepper, and salt. Mix gently until evenly coated.

Transfer mixture to prepared baking dish, spreading evenly. In a small bowl, mix panko breadcrumbs, melted butter, and remaining 1/2 cup cheddar cheese.

Sprinkle breadcrumb mixture evenly over zucchini. Bake uncovered for 35-40 minutes until zucchini is tender and top is golden brown.

Let rest for 5-10 minutes before serving. Garnish with fresh parsley if desired.