Fill A Large Pot With Lightly Salted Water And Bring To A Rolling Boil Over High Heat. Once The Water Is Boiling, Stir In The Macaroni, And Return To A Boil. Cook The Pasta Uncovered, Stirring Occasionally, Until The Pasta Has Cooked Through, But Is Still Firm To The Bite, About 8 Minutes. Drain Well In A Colander Set In The Sink. Melt The Butter In A Skillet Over Medium Heat. Stir In The Onion; Cook And Stir Until The Onion Has Softened And Turned Translucent, About 5 Minutes. Reduce Heat To Low, Then Stir In The Flour, Mustard, And Salt Until Thoroughly Combined. Gradually Stir In The Milk, Then Increase Heat To Medium-High And Simmer Until Thickened, Stirring Constantly, About 3 Minutes. Stir In The Cheddar Cheese And Black Pepper Until Cheese Is Melted And The Sauce Is Smooth, About 3 Minutes. Stir In The Macaroni Until Well-Coated And Heated Through.