Cook macaroni in salted water. Drain and toss with a little oil or butter to prevent clumping. Set aside.
Mix the flour, salt, pepper, and dry mustard in a small bowl. Grate the cheese using a box grater.
Melt the butter over medium-low heat in a skillet or the same pot you used to cook the pasta. Whisk in the flour mixture. Cook for about 2 minutes, whisking constantly, until the mixture turns golden brown.
Gradually whisk in the milk, a quarter cup at a time, mixing well after each addition. Continue cooking and stirring until the mixture is thick enough to coat the back of a spoon, about 1 ½ minutes.
Remove the pot from the heat and stir in the shredded cheese until it melts and the sauce is smooth.
Add the cooked macaroni and stir until the pasta is evenly coated. Serve hot.