Heat a saucepan over medium-low heat. Add the butter and allow it to melt.
Add the flour. Continue to stir until the flour has absorbed all of the butter.
Continue to cook for 2-3 minutes until the mixture smells slightly nutty but the color is still blonde.
Slowly drizzle in warm milk, continually whisking until all of the milk is absorbed before adding any more. Continue to whisk until all of the milk has been incorporated.
Continue to whisk the sauce, allowing it to thicken. It should hug the back fo a spoon.
If the sauce seems too thick, drizzle in a little extra milk.
Add the salt, pepper, and nutmeg. Stir to combine.