Pour the milk into a small saucepan and bring to just below a boil over medium heat. Meanwhile, melt the butter in another small saucepan over low heat. Add the flour and cook, stirring, for about 5 minutes; do not let the mixture color. Whisk in the hot milk and continue to whisk until thickened and all the raw flour taste is gone, 8 to 10 minutes. Season with salt, pepper, and nutmeg.