Place milk, cream cheese, flour, and salt in a blender and blend until smooth.
In a non-stick sauce pan over medium heat, melt butter and add garlic. Let the garlic saute for about 30 seconds.
Add the milk mixture to the pan. Stir constantly for about 3 or 4 minutes or until it just comes to a simmer. Keep stirring and let it cook for a few minutes more. It should be much thicker now.
When it's nice and thickened turn heat to low. Add the cheese, stir it up to melt and then remove pan from heat and cover.
Let stand for 5-10 minutes before using. It will continue to thicken upon standing. Season with additional salt if needed.
Notes
Store leftovers in the fridge. The sauce will thicken almost into a solid. Just re-heat and add a little milk and it will be back to normal again.