When making a roux for Espagnole sauce, I always use clarified butter and flour. It is my understanding as I follow the process for making the Espagnole sauce that the scum that I am skimming off the top of the stock mixed with the roux is the butter coming out of the sauce. So then, is not my Espagnole sauce pretty much fat free when finished? I rarely see any solidified fat in the jars that I keep in the fridge or after canning them. I thought that the butter was like a "carrier" of the starch component (flour) so that the flour would be evenly dispersed throughout the sauce thus thickening it. Is this an incorrect understanding?
When I roast a pork tenderloin in a rack in a heavy roasting pan, I always deglaze the roasting pan into a sauce pan, and just add flour to create a roux from the pork fat and flour. It seems to work pretty good, and I don't think of the resulting "sauce" as "gravy".
I cannot recall for sure, but thought that the definition of a roux was "fat and starch", so would not any combination of fat and starch make a proper roux? It would therefore seem plausible to choose which combination of fats and starch to make a roux based on the nature of the dish for which the roux is destined and the resulting taste created by the choices.
I am confident that someone will set me straight on my thinking here!