Melt 2 tbsp butter over medium heat in a medium saucepan. Stir in the flour and cook for 1 minute, stirring constantly. Add milk, salt, mustard, and pepper. Continue stirring until the sauce thickens and begins to boil.
Meanwhile, bring to a boil a saucepan of slightly salted water. Add the macaroni and cook for 8 to 10 minutes, or until pasta is al dente. Drain.
Add the cheese to the milk mixture and stir until melted. Mix the pasta with the sauce and pour it all into a gratin dish.
In a bowl, mix together bread crumbs and 1 tbsp melted butter.
Add the bread crumb mixture to the top of the macaroni.
Bake in the preheated oven for 30 minutes, or until the mixture begins to bubble. Let cool 10 minutes before serving.