Put the stock into a large saucepan. Add the beef, garlic, coriander roots, cinnamon, star anise, soy sauce, fish sauce and sugar. Bring to the boil and simmer for 30 minutes. Skim off the scum occasionally.
In the meantime, put the beansprouts into a serving bowl. Pour the hot soup over the beansprouts, garnish with chopped spring onion and coriander and serve.