So I found this recipe to do Roux for potato soup.

[sir pball]

11. There is. Also to bind up the bacon fat.

I usually blitz in a good amount of butter at the very end of my potato soup, after it's all done…throw in cold cubes while buzzing with a stick blender, usually a stick or so for a 4qt batch. Gives it an indescribable velvet richness.

I bet doing that with the bacon grease would be pretty amazing, no need to fiddle around with a roux at all.