Make Roux for Gravy and Freeze for Later Use
When you make stock, use the resulting fat to make roux. Then freeze the roux in small disks for later use. You'll always have a richly flavored roux base on hand for making gravy and sauces.
When making gravy, just add a frozen disk or two. The roux melts slowly, so there are no lumps, and it flavors and thickens at the same time. Here’s how to do it: Chill stock overnight. Transfer congealed fat to a saucepan to make roux. Cool and scoop tablespoons of roux onto a parchment-lined baking sheet; freeze. Once frozen, store roux disks in a plastic freezer bag.