Preheat the oven to 450 degrees.

Steam the broccoli in a steamer basket or in a microwave steamer bag until tender crisp. Place the broccoli in a 9 x 9 baking dish.

Melt 2 tablespoons of butter in a medium microwave safe dish. Remove from the microwave and add the breadcrumbs and Parmesan cheese. Mix well and set aside.

In a medium saucepan melt the remaining butter. Add the onion and garlic and cook for 3 minutes.

Stir in the flour and cook for 2 minutes.

Add in the milk and cook, stirring frequently, until the mixture thickens, about 4 minutes.

Stir in the cheddar cheese, pepper, and mustard and cook until the cheese is melted, about 2 minutes.

Pour the cheese sauce over top of the broccoli and mix to coat. Sprinkle with the breadcrumb mixture.

Bake for 20 minutes. Remove from oven and cool for 5 minutes before serving.