Preheat your oven to 375°F and lightly grease an 8x8-inch baking dish with butter or non-stick spray.
Bring a large pot of salted water to a boil, add the broccoli florets, and cook for 3 minutes until bright green and crisp-tender. Immediately drain and rinse under cold water to stop the cooking process.
In a large mixing bowl, combine the mayonnaise, sour cream, beaten egg, chopped onion, garlic powder, salt, and pepper. Fold in the blanched broccoli and 1 1/2 cups of the shredded cheddar cheese until everything is evenly coated.
Transfer the broccoli mixture to the prepared baking dish and spread it into an even layer. Sprinkle the remaining 1/2 cup of cheddar cheese evenly over the top.
In a small bowl, stir together the panko bread crumbs and melted butter. Sprinkle this buttered panko mixture over the cheese, followed by a dusting of paprika.
Bake for 25-30 minutes, or until the casserole is bubbly around the edges and the topping is a deep golden brown. Let it rest for 5 minutes before serving to allow it to set.
