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, then 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 Worchester 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.
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. Serve each Salisbury steak with mushroom and onion gravy on top of egg noodles or mashed potatoes. Enjoy.
Omit the extra tablespoon of cornstarch and water if your gravy is already at your desired thickness.