Combine the beef, barley, mushrooms, celery, carrots, leeks, thyme, beef broth and soy sauce in a slow cooker. Add 1 cup water, 1 teaspoon salt and 1/4 teaspoon pepper. Cover and cook on low, undisturbed, 8 hours. Uncover and skim off any excess fat. Transfer the beef to a cutting board, let cool slightly and slice or shred by hand into bite-size pieces. Thin the vegetable-barley mixture in the slow cooker with some water, if desired. Divide among shallow bowls and top with the beef. Serve with horseradish, if desired.