Bring a medium pan of water to a boil, add the noodles and cook for two minutes. Drain, divide between two bowls, add the oil and mix well to stop the noodles sticking together.
Put the stock in the same pan and bring to a boil. Add the lemongrass, chillies, soy sauce, sugar and chilli jam or oil (if using, for extra heat and colour). Mix well and cook for two to three minutes.
Add the mushrooms, cook for five minutes, then add the lime leaves, beansprouts, most of the coriander and the lime juice. Stir well for two minutes, then taste and adjust the flavour, adding more soy or lime juice as you prefer.
Take off the heat and pour evenly over the noodles. Top each bowl with some Thai basil, a little of the reserved coriander and the chilli flakes, if using. Discard the lemongrass pieces as you eat (they will be woody, but will have added a lovely aroma).
