In a large skillet brown up the ground beef and onion until the beef is browned. Drain off any grease. Add your beef broth, ketchup, Worcestershire sauce and black pepper, stir together. If cooking on the stove only skip the next step and move to step 3.
To a 6 quart slow cooker add your beef broth, ketchup, Worcestershire sauce and black pepper. Let cook on low 4-6 hours.
Mix the cornstarch with the cold water, add into pot and stir. Add the cheese slices on top and stir in once they are melted. Build your subs with lettuce, tomato and the beef mixture.