For the dumplings, mix the suet, flour, salt and baking powder together in a bowl and stir in the horseradish and parsley, if using. Gradually add milk or water to the mixture, mixing well to form a soft dough. With floured hands, roll small pieces of the dough into dumpling-sized balls. Set aside.
For the stew, place the beef brisket into a large saucepan and scatter around the onions, carrots, celery, potatoes, bay leaf and parsley. Add enough water or beef stock to cover, then bring to the boil.
Cover with a lid, reduce the heat to its lowest and simmer for at least 1-2 hours, or until the meat is tender. Add the dumplings to the stew 20 minutes before the stew is finished cooking. Cover with a lid and do not lift again until the end of the cooking time, or until the dumplings are cooked through.
To serve, spoon the stew and dumplings into bowls.
