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.
Step 2
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.
Step 3
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.
