Melt butter. Once bubbly, add flour and mix until a paste is formed. Slowly add milk, stirring until thickens. Remove from heat and add mustard, salt, pepper and cheese. Mix well then add pasta and combine together.
You can serve as is, or to create a crunchy top coating, place pasta in baking dish and top with panko crumbs and Parmesan cheese. Bake on 350° for about 20 minutes until crust is crispy and cheese is bubbling.