Step 1Pour the stock into a medium saucepan, place over a high heat and add the dried porcini, ginger, garlic, spices, lemongrass and white spring onions. Add the chicken thighs, bring to a low simmer and cook gently for 25 minutes or until the chicken is tender.
Step 2Meanwhile, put the carrot julienne and rice noodles into a large heatproof bowl, pour on boiling water to cover generously and leave to soften for 10 minutes. Finely shred the green spring onions for the garnish.
Step 3Pour the broth through a strainer into a jug, remove the chicken from the strainer and set aside for later; discard the rest of the contents. Return the flavoured broth to the pan and place over a medium-high heat.
Step 4Bring back to the boil and let bubble for 5 minutes to reduce and intensify the flavour. Stir in the fish sauce and lime juice. Season the soup with a pinch of sea salt if needed and a small pinch of white pepper.
Step 5Shred the chicken, discarding the bones, then add to the broth to warm through for 2 minutes. Drain the noodles and carrot and share between warmed bowls. Scoop the chicken out of the pan and add to the bowls. Ladle over the broth and finish with the spring onions, beansprouts, herbs, chilli and lime wedges.
TO FREEZE Allow the strained broth to cool at the end of stage 4 then freeze in a suitable container. Shred the chicken and freeze separately. Defrost both at room temperature, then reheat the broth thoroughly in a pan over a medium heat. Continue as above.