Step 1: Add the steaks to the slow cooker - Start by placing the cube steaks into an eight quart slow cooker. Lay them flat as best you can. Sprinkle the onion powder and garlic powder evenly over the steaks. This gives the meat flavor right from the start.
Step 2: Add onions and mushrooms - Layer the sliced onions and mushrooms over the top of the seasoned cube steaks. Do not worry about stirring yet. As everything cooks, the vegetables will release moisture and blend right into the gravy.
Step 3: Mix the gravy - In a small bowl, combine the beef broth, cream of mushroom soup, and brown gravy packet. Stir until the mixture is smooth and thick. This step only takes a minute but makes a big difference in texture later.
Step 4: Pour and cook - Pour the gravy mixture evenly over the steaks, onions, and mushrooms. Cover the slow cooker and cook on low for six hours. The steaks will become tender and the gravy will thicken beautifully.
Step 5: Serve and enjoy - Once fully cooked, gently stir everything together. Serve the Salisbury steak and mushrooms over mashed potatoes, making sure to spoon plenty of gravy over the top. Sit down, take a breath, and enjoy a meal that tastes like it took far more effort than it did.