Cook the elbow macaroni according to the package instructions. Drain and set aside.
In a large skillet, melt the butter over medium heat. Once melted, add the flour and whisk until well combined. Cook for about 1 minute to get rid of the raw flour taste.
Slowly pour in the milk, whisking constantly to avoid any lumps. Continue cooking and whisking until the mixture thickens, about 5 minutes.
Reduce the heat to low and add the shredded cheese to the skillet. Stir until the cheese is melted and the sauce is smooth.
Stir in the Dijon mustard, salt, and pepper. Taste and adjust the seasoning if needed.
Add the cooked macaroni to the skillet and toss until every noodle is coated in the cheese sauce.
Cook for an additional 2-3 minutes to allow the flavors to meld together.
Remove from heat and let it cool slightly before serving.