Heat the stock in a pan and flavour with a few aromatic veg and spices if you have any to hand. Here I used some a large piece of ginger, a clove of garlic and half an onion - all charred in a dry pan to get the flavours going. In terms of spices I used a stick of cinnamon, star anise and a few cloves. (This is not essential, but does add wonderful fragrance and depth of flavour). Add it all to the pot and bring to a simmer with a splash of fish sauce and soy sauce. Meanwhile, cook the egg noodles in a pot of boiling water*.
At the last minute, while the noodles are cooking, drop in the prawns (I used frozen cooked prawns from the supermarket) and some greens, such as mangetout, sugarsnap or just plain old peas from the freezer.
Add the noodles to a large bowl, pour over the hot soup with the and prawns and greens and top it off with some chilli sauce and a squeeze of lime.
[* Many recipes suggest cooking the noodles separately in this way, 'refreshing' them by running under cold water and then adding them to the bowl before pouring over the steaming soup. Other wise you could just drop a nest of dried noodles straight into the hot stock. Having tried both methods, I'm not sure I can tell the difference.]