In a saucepan over medium low heat, melt the butter and add then add the garlic. Cook a few minutes and then stir in the flour. A flat bottomed wooden spoon or a whisk works well for this.
Cook the flour and butter over medium heat for a few minutes, stirring most of the time.
Whisk or stir in the milk slowly so you don't get any lumps.
Bring to a simmer and cook until thickened a bit.
Take off the heat and stir in the Parmesan cheese and herbs, if using.