Pour the chicken stock into a large saucepan, add the lemongrass, ginger, chilli and garlic and bring to the boil. Add the chicken, reduce the heat and simmer for about 15 minutes until the chicken is cooked.
Lift out the chicken with a slotted spoon, put onto a chopping board and allow to cool. Leave the pan of stock on the heat.
Break or cut the noodles into the hot stock, then add the baby corn, mushrooms, soy sauce and coconut milk. Bring the pan back to simmering temperature, then cook for 3 minutes.
Meanwhile, cut the chicken into fine shreds. Stir the chicken into the soup with the lime zest and juice and the pak choy and simmer gently for 2 minutes until the pak choy has wilted.
Ladle the soup into four soup bowls and scatter with spring onions and coriander leaves. Provide a spoon and fork for ease when eating.