1. Place first 3 ingredients on a double layer of cheesecloth. Gather edges of cheesecloth together; tie securely.
2. Place brisket in an 8-quart stockpot; cover with 2 quarts water. Bring to a simmer; cover, reduce heat, and simmer 1 hour. Add onions, carrots, and cheesecloth bag; cover and simmer 30 minutes. Add cabbage and potatoes; cover and cook an additional 30 minutes or until vegetables are tender. Remove vegetables and brisket from pan; cut brisket across grain into thin slices. Discard cheesecloth bag and half of cooking liquid. Serve with horseradish, if desired.