Preheat your oven to 350°F (175°C) and grease your 9x13-inch baking dish with a little butter or non-stick spray.
Prepare the broccoli: If you're using fresh broccoli, steam or blanch it until it's tender but still bright green (about 4-5 minutes). If you're using frozen broccoli, thaw and drain it well to remove any excess moisture.
Mix the casserole base: In a large mixing bowl, combine the cream of mushroom soup, mayonnaise, beaten egg, milk, garlic powder, onion powder, salt, and black pepper. Stir everything together until it's smooth.
Add the broccoli: Once the broccoli is ready, stir it into the soup mixture until well combined.
Add the cheese: Stir in 1 1/2 cups of the grated cheddar cheese (save the remaining 1/2 cup for topping later) into the mixture.
Transfer to the baking dish: Pour the broccoli and cheese mixture into the prepared baking dish. Spread it evenly.
Add the topping: In a small bowl, mix the panko breadcrumbs with the melted butter and the remaining 1/2 cup of cheddar cheese. Sprinkle this mixture evenly over the top of the casserole.
Bake: Place the casserole in the oven and bake for 30-40 minutes, or until the top is golden brown and the casserole is bubbly.
Serve: Let it cool for a few minutes before serving. Enjoy the warm, cheesy goodness!