Melt butter on low in small saucepan.  Add flour and stir to coat.  The butter-flour mixture is a roux, so cook the roux for a few minutes until it is fragrant.

Add the milk to the pan and whisk together until the flour mixture melts into the milk. Turn heat up to medium until milk is hot and continue to whisk until the mixture is near boiling.  Once it starts to thicken, turn the heat down to a simmer.   Use a rubber spatula to scrape the bottom of the pan, so the flour does not scorch.

Once the sauce has thickened, add a pinch of nutmeg and remove it from the heat. Refrigerate until you are ready to use in your recipe. Can also be stored in freezer and thawed out when needed.