Spray a large 6-quart oval slow cooker with cooking spray. Then add the sliced mushrooms and onions to the bottom.
Next, prepare the ground beef mixture. Combine the ground beef, steak seasoning, 1 tablespoon of Worcestershire sauce, dried onion flakes, parsley, breadcrumbs and egg together. Mix well.
Divide the mixture into 4-6 patties. Form each patty into an oval shape, pressing thin, and then place in the slow cooker (make sure you spread evenly in the slow cooker and do not stack).
Mix together the 2 cups of beef broth, 2-tablespoons Worchestire sauce, 2 tablespoons cornstarch, and black pepper. Whisk until smooth then pour on top of beef patties. Place the lid on the slow cooker and cook for 4 hours on high.
*Half way through the cooking process flip the patties over and mix the gravy. If you are not home to do this, do not worry. The recipe will still come out great. I just like to keep the steaks moist with the gravy.
Once the Salisbury steak is done cooking, remove the steaks from the slow cooker.
In a small bowl add one-tablespoon cornstarch with 2 tablespoons water. Stir the cornstarch water mixture into the gravy in the slow cooker. Mix until thickened. Place the steaks back in the slow cooker. Enjoy.