Place the stock, fish sauce, lime juice, ginger and cinnamon in a large saucepan over high heat and bring to the boil. Add the noodles and cook for 6 minutes. Add the bok choy and cook for a further 2 minutes. Remove from heat and add the beef. Top soup with the chilli, green onion, basil and mint to serve. Serves 4.
+ Freeze the beef for 20 minutes to make it easier to cut into thin slices. The thin slices will cook quickly from the heat of the soup.