In a medium sauce pan over medium-low heat, melt the butter.
Add in the garlic (if using) and cook for 1-2 minutes, constantly stirring. They should become fragrant and lighlty golden.
Whisk in the flour. Cook for 1-2 minutes, stirring frequently. The mixture will turn into a roux. The color will turn pale to a light brown, almost like peanut butter.
Then slowly and carefully, whisk in the milk and heavy cream. There should be no lumps.
Continue to cook, stirring often, for 5-10 minutes, or until the mixture begins to thicken. If you insert a spoon into the mix and swipe the back of it, the sauce should stay in place and not be loose.
Stir in the parmesan cheese and nutmeg. Stir for 1-2 more minutes or until the mixture is well combined and thickened. Season with salt and pepper.
Remove from heat. From this point, this sauce can be tossed with your preferred pasta or for another dish.
Notes
To store: Seal in a tight jar or container and store in the fridge for up to a week.