Preheat your oven to 375 degrees.
Cook pasta according to package directions and set aside.
Melt butter in a medium saucepan. Remove from heat.
Stir in flour, salt, pepper, and mustard and stir until smooth.
Now add the milk and stir constantly until boiling.
Remove from heat and stir in TWO cups of cheese.
Reduce heat to low and continue to stir until everything is combined.
Stir in pasta and pour into a baking dish.
Sprinkle top with remaining cheese.
Bake 15-20 minutes or until the cheese on top is golden brown and the casserole is bubbly.