Melt the butter over medium high heat in a heavy bottomed medium sized saucepan.
Add salt, pepper and flour to the pan. Whisk until smooth and the rue turns a pale brown.
Slowly pour milk and whisk until combined.
Add 1/2 cup of Parmesan cheese, continue to whisk until cheese melts into the mixture and becomes smooth.
Dip a spoon in a taste. Adjust seasoning and cheese if needed. If the Parmesan flavoring is too weak add 1/4 to 1/2 more to the pot and continue to whisk.
Continue to cook over medium heat, whisking until the sauce becomes thick.