Boil the pasta according to the package instructions.
Melt the butter in a deep skillet over medium-low heat. Add in the flour, whisking it constantly until it starts to bubble lightly and smell a bit nutty (about 2-3 minutes).
Slowly whisk in the milk and let the mixture cook for about 5 minutes until the texture turns custard-like. Whisk in the dijon mustard.
Stir in the cheese until it's all dissolved and creamy.
Add in the cooked macaroni and stir until well-combined. Dig in and enjoy!