Heat the oil in a large saucepan and add the spring onion and mushrooms. Fry gently for 2-3 minutes until softened.
Meanwhile, separate the green leaves from the white stalks of the pak choi. Slice the stalks into 1cm pieces and add to the pan along with the ginger, frying for a further minute until starting to soften.
Add the boiling water and stock powder to the pan. Bring to the boil, then add the noodles and cook for 5-6 minutes until tender.
Finely slice the green tops of the pak choi.
Once the noodles are tender, add the pak choi leaves and miso paste to the pan, and simmer gently for a minute or two.
Pour into a bowl, squeeze over the lemon juice, and eat immediately!