First I saw this thread, but you can't get an explanation better than Chef Jimmy's.
I had some of that left over a while back, and I put it in an ice cube tray, froze it, put the cubes in a vacuum pack & stuck it in the freezer.
I did a post on it, but I didn't include all of the know how that Chef Jimmy pours out of that knowledgeable cooking brain of his.
I just figured it was left over Au Jus that thickened up real nice.
If anyone's interested, here is the link to that thread:
Thanks Dave & Jimmy!!!