Place beef in a large Dutch oven and cover with water. Add onion, optional beer, thyme and parsley. Bring to a boil over high heat. Meanwhile, place bay leaf, peppercorns, mustard seed and cloves into herb ball or piece of tied cheesecloth. Add to pot; reduce heat to simmer. Cover and cook for about 3-1/2 hours or until meat is very tender (takes about 1 hour per pound of meat). Add cabbage, carrots and parsnips. Simmer for 20-30 minutes until vegetables are tender. Remove the meat and cut into slices. Place on center of a large platter. Strain the vegetables and place around beef. Add pepper over all. Pass horseradish sauce and, if desired, some of the broth.