Place a large pot of water over high heat and bring it to a boil. Once at boil, add some salt and the pasta, and cook just shy of al dente, according to package directions.
While the pasta is cooking, heat a medium-size skillet over medium heat and add the butter. When the butter has melted add the flour and cook about a minute. Whisk in the milk, season with salt, ground black pepper, and ground mustard, and bring it to a bubble. Turn the burner off and stir in the cheese in a figure eight motion until its melted.
When the pasta is done, drain it well and combine with sauce. Pour into baking dish, add cubed cheddar to the top, and top with bread crumbs. Cover with foil and bake for 10 minutes, remove foil and bake for an additional 15 minutes or until golden brown on top