Lightly spray the inside of a 6-quart slow cooker with non-stick spray.
Add in the packet of au jus, both cans of soup and the water. Stir well.
Season the cube steaks (I'd go light on the salt, but heavier on the pepper) and add to the slow cooker.
Cover and cook on low for 6-8 hours. If desired, flip the steaks once halfway through the cooking time to get the sauce well-coated onto the cube steaks.
In a small glass or bowl, whisk together the cornstarch and water until well combined.
Carefully remove the steaks from the slow cooker and place on a plate.