Brown the brisket in hot lard, pour off drippings. Add salt, mustard seeds, bay leaf, peppercorns, garlic and 12/12 cups of water, cover and simmer for 2 hours. Turn the brisket, add onions, cover and simmer for 1 hour longer or until the meat is tender,
Remove the meat to a serving platter, let stand for 15 minutes. Remove onions, keep warm. Discard the bay leaf and peppercorns. Measure the cooking liquid, adding water, if needed, to make 2 1/2 cups. Blend the flour with an additional 1/2 cup of water; add flour mixture to cooking liquid and cook, stirring constantly, until thickened. Stir in horseradish. Sprinkle brisket and onions with paprika, if desired. Slice the brisket across the grain; serve with onions and gravy. Serves 10 -12
