Melt the butter in a heavy saucepan over low heat.
Stir in the flour with a wire whisk and cook over low heat for about three minutes while stirring constantly. (Do not allow the mixture to brown).
Gradually stir in the milk while whisking constantly and cook over low heat for about 3-5 more minutes (the sauce should begin to thicken).
Remove from heat and stir in the cheese.
Whisk until the cheese has melted and smooth.
Season to taste.