For the soup to have a deep flavour, it is really important to sear off the oxtail meat until dark brown.
If the soup is too thin, stir in some cornstarch mixed with cold water and bring to the boil again.
The tail being a tough cut of meat, it needs long and slow cooking to release its flavours and become meltingly tender.
It is believed that oxtail soup originated in France during the French Revolution, when the political turmoil resulted in widespread poverty. Cowhides were send from slaughterhouses to tanneries with the oxtail still on, deemed useless. One day, a noblemen saw the pile of discarded oxtails and asked for them. He made the very first oxtail soup. The French started to use this cheap cut of meat for a soup, up to the point that the tanners put a price on them being annoyed by the demand for the tails. The soup became popular in England when the French people migrated there.