Warm 1 1/2 cup Milk in skillet just to low simmer. Mix Corn Starch in 1/2 cup Cold Water to make slurry, add to pan and stir. increase heat and stir until thickened, Corn Starch needs to come to rolling simmer or low boil to thicken. Then reduce heat to low simmer.
Add the Sour Cream and Cream Cheese if using. Add Butter. and remaining ingredients except salt and pepper. Stir to incorporate. Note: 2 tablespoons Olive Oil could be added in lieu of some of the butter.
Simmer for ten or fifteen minutes. Stir occasionally then taste. Parmesan is salty so should require very little additional salt. After sampling add salt and pepper to your taste.
This freezes well. If re-heating a little Milk may be used for the consistency of your liking.
The Topper -- I like cooked Shrimp or Scallops heaped on my Alfredo. Sauteed Chicken works also.