Add the ground beef, onion soup mix, Worcestershire sauce, steak seasoning, egg, and bread crumbs to a large mixing bowl.
Brown the beef patties in a hot skillet over medium-high heat for 2-3 mins on each side. You do not need to cook them through.
Pour the gravy over the beef patties in the slow cooker. Put the lid on the slow cooker and cook the Salisbury steaks as directed.