Add all of the ingredients to a saucepan and simmer on medium-low for about 20 minutes, or until the sauce has thickened enough to serve. Stir constantly.
Serve immediately.It is possible to reconstitute cheese sauces after it re-solidifies at room temperature. Just keep it at a long temperature, stir constantly, and slowly bring up the heat. Bringing up the heat too quickly can cause the cheese to separate and you may need to add a bit more sour cream if the sauce is too thick at this point.