In a 4-quart Dutch Oven brown oxtail and onion in hot oil for several minutes. Add water, salt and peppercorns; simmer uncovered for about 2 hours. Cover and continue to simmer for 3 additional hours. Add the parsley, carrots, celery, bay leaf, tomatoes, and thyme; continue simmering for 30 minutes longer or until the vegetables are tender. Strain stock and refrigerate for an hour or more. In a blender puree the edible meat and vegetables and reserve. Remove fat from top of stock and reheat. In a large, dry frypan brown flour over high heat. Cool slightly. Add the butter or margarine, blend. A little at a time, add the stock and vegetables. Correct seasoning and add Madeira just before serving.