whole milk, at room temperature
Melt the butter in a small saucepan over medium,
Whisk in the flour and cook for a minute. Be sure it doesn't brown.
Slowly whisk in the milk.
Raise the temperature and continue whisking until it starts to thicken. About 5 minutes.
Add the salt and pepper to taste.
I use about 1/2 teaspoon salt and a generous amount of pepper.