Place the stock in a large saucepan. Add the galangal, shallots, ginger, chilli, lime leaves and the finely chopped coriander stalks. Bash the lemongrass with the back of your knife to bruise it, chop each stalk into 4 pieces and add to the stock. Bring to a simmer and cook gently for 10 minutes, until aromatic and well flavoured.