The Food Show
Heat the butter, add the flour, and stir as if you're making a roux. Cook until the texture changes.
Whisk in the milk and nutmeg until it takes on the texture of very light mashed potatoes. At first, it doesn't look likely for that to happen, but then--as if by magic--it does.
1 stick butter
1 cup flour
1 1/2 cups warm milk
Tiniest pinch of nutmeg (optional)