Add first 6 ingredients to your crock pot and stir to combine. If mixture seems too thick add a little bit more milk.
Turn crock pot on high and cook for 4 hours or on low for 8 hours. If you let it cook longer on either setting, check on it to make sure it is not too thick. You want to make sure that your sauce is a little thinner as it will thicken up with next step.
Before serving, mix in Sour Cream. This should give your sauce the smooth consistency of gravy.