A cornstarch slurry is an ideal way to thicken gravy for cube steak. If you don’t have any cornstarch on hand, you can substitute flour (instant-blending flour is ideal) to make a roux. Failing that, a gravy mix is a fallback option. Simply follow the package directions. Note that you may need to use less salt than the recipe calls for, as packaged gravies tend to be quite salty on their own.