Let the Cube steaks come up to room temperature, which should take under 15 minutes. Sprinkle with a pinch of salt and pepper. If searing, heat the oil in a cast iron pan and brown the meat for 1 minute per side
Set the crockpot on medium heat. Place the mushrooms and onions in the bottom of the insert. Layer the meat on top. Pour in the condensed soup, not adding any other liquid. Sprinkle the onion soup mix in on top and mix it together, flipping the meat so some of the liquid reaches the bottom of the crockpot
Cook on medium for 3 to 4 hours, checking for when the steaks become easily pierced by a fork. Leave the crockpot on low for up to 2 hours more before serving