Cook the onion and all the ingredients up to and including the cinnamon over a medium heat for approx. 30 mins. Strain the stock, return to the pan and bring to the boil.
Prepare the noodles according to the packet instructions, drain and divide with the beef into 2 large soup bowls. Pour over the hot stock and leave to infuse for approx. 1 min. Add the spring onion, lime and bean sprouts, serve with coriander.