Preheat your oven to 375°F (190°C).
Lightly grease a 9x13-inch baking dish.
Steam the broccoli florets until they are tender but still crisp, about 4-5 minutes.
Drain well and set aside.
In a medium saucepan, melt the butter over medium heat.
Add the flour and stir to make a roux.
Cook for about 2 minutes, until the mixture is bubbly and without changing the color.
Gradually whisk in the milk, ensuring to break up any lumps.
Cook, stirring constantly, until the mixture thickens, about 5-7 minutes.
Remove from heat.
Stir in the cheddar cheese, salt, pepper, and nutmeg until the cheese is melted and the sauce is smooth.
Combine the cooked broccoli and the cheese sauce, stirring gently to coat the broccoli evenly.
Transfer to the prepared baking dish.
In a small bowl, mix the panko breadcrumbs and the grated Parmesan cheese.
Sprinkle this mixture evenly over the top of the broccoli and cheese mixture.
Bake in the preheated oven for 20-25 minutes, or until the topping is golden brown and the sauce is bubbly.
Serve hot!