Cook pasta according to package directions. Drain.
While pasta is cooking, in a large pot over medium high heat, melt butter. Whisk in flour and cook a couple of minutes.
Pour in the cold milk and whisk to get rid of the lumps. Bring to a quick boil, then reduce heat and allow to simmer, whisking frequently, until sauce has begun to thicken, about 5 to 10 minutes. Remove from heat.
Slowly stir in shredded cheese until it has melted completely. Stir in the mustard, salt and pepper.
Fold in the cooked pasta and enjoy!
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Notes
Calories are automatically generated. For best results calculate based on your exact ingredients.Note: sauce will thicken as it cools, whisk in additional milk or water as needed when reheating.