In a saucepan over medium heat, melt the butter. Whisk the flour and garlic salt into the pan until you've got a smooth mixture. Carefully beat the half and half into the mix until it has completely incorporated. Stir in the garlic, parsley, and Parmesan. Make sure to whisk continually. Get the whole thing simmering and cook, stirring regularly, until the sauce is nice and thick, 4 to 5 minutes. You can use it right now, or cover it up and save it for up to a day.