Preheat oven to 375°F
Cook pasta al dente.
Melt butter in saucepan.
Add flour, stirring together to make a roux.
Slowly add milk, ensuring that the consistency stays thick.
Stir in onion, cheese, mustard, nutmeg, salt, and pepper.
Combine sauce with pasta.
Sprinkle cheese on top.
Toss butter and bread crumbs together and then sprinkle bread crumbs on top.
Bake at 375F for 30 minutes or until golden on top.