1. Place the onion, carrots, celery, cabbage, bay leaf, peppercorns, and beef broth in a 3-quart saucepan. Cover and bring to a boil. Reduce the heat and simmer for 8 to 10 minutes, or until the cabbage is crisp-tender. (Cook about 2 minutes longer if you like very tender cabbage.) Add the salt and pepper.
2. Divide the beef equally among 6 soup bowls and ladle the soup over it. Top each serving with 1 tablespoon of the sour cream if desired, or serve the sour cream on the side.