Peel the onions and carrots. Trim and rinse the leek well. Set aside half of the onions, carrots and leek. Cut the remaining vegetables into large cubes.
Dredge the oxtail pieces in flour. Heat the clarified butter in a Dutch oven and saute the oxtail over medium heat until golden brown on all sides. Add the diced vegetables, herbs and allspice, deglaze with Madeira and 2.5 liters (approximately 2 1/2 quarts) of water and bring to a boil. Reduce to a simmer and cook over low heat, stirring occasionally until the oxtails are tender, about 2 1/2 hours.
Cut the remaining carrots, onions and leek into julienne and blanch briefly in a pot of boiling salted water. Drain, cover and refrigerate.
Remove the oxtail from the broth, let cool, then remove the meat from the bones and cut into small cubes. Strain the broth and refrigerate both the broth and meat overnight. Skim the fat from the top of the broth.
Place the skimmed broth in a saucepan and bring to a boil. Add the julienned vegetables and meat to the broth, season with salt and pepper and serve garnished with the herbs.