Fry the oxtail pieces and the vegetables in the fat for a few minutes until lightly browned. Add the stock and bouquet garni. Bring to the boil, cover and simmer for 1 1/2 - 2 hours until the meat is tender.
Strain the stock, cut all the meat from the bones and return the stock and meat to the pan. Bring to the boil. Liquidise.
Mix the flour and port together and add to the soup. Simmer for 5 minutes before serving.