Halve the lemongrass, peel the ginger, and peel and roughly chop the garlic and shallots. Pick the coriander leaves and finely chop the stalks.
Place the lemongrass, ginger, ground coriander, garlic, chillies (leave the seeds in, unless you don’t like it hot), shallots, lime leaves and coriander stalks in a food processor with a good drizzle of oil and blitz to a paste. Add the soy sauce and blitz again.
Heat a little oil in a big saucepan and fry the paste for 1 minute, stirring all the time. Stir in the stock and coconut milk. Bring to the boil, then reduce the heat and simmer for 15 minutes.
Bring a pan of salted water to the boil. Add the noodles and beansprouts and cook for 3 minutes. Drain and refresh in cold water, then add to the soup with the veg to cook for the last 5 minutes.
Transfer to bowls, top with coriander leaves, and cut the limes into wedges for squeezing over.