In a deep bottom saucepan, melt the butter over medium heat. Add mustard and flour. Whisk briskly for about 2 minutes to cook the flour. Pour in milk, whisking continuously to prevent any lumps. Keep whisking until the sauce thickens enough to coat the back of a spoon, about 2 to 3 minutes. Stir in cheese until smooth and add salt and pepper to taste.
Cook the pasta until just al dente. Drain the water and pour the pasta into the pot with the cheese sauce. Transfer the mixture to a casserole dish. Sprinkle evenly with breadcrumbs
In a preheated 350°F oven, bake for 30 minutes or until bread crumbs are toasted and the mac and cheese is bubbly. Top with chopped parsley if desired.
When cooking your pasta, be sure to cook it just until al denté. When it's baked in the oven, the macaroni will continue to cook and you don't want to end up with mush.