Heat the water to boiling point.
Meanwhile, clean the mushrooms and slice them into bite-sized pieces. Tear or cut the nori sheet into small pieces.
Chop the mangetout and slice the spring onions lengthwise. When the stock has reached boiling point, stir in the soy sauce and miso paste, then add the mushroom pieces and simmer over low heat for 5 minutes. Add the spring onion, mangetout and nori pieces. Cook for a couple of minutes.
Meanwhile, cook the noodles in another pan, following the packet instructions. This should take about 5 minutes.
Drain the noodles, then serve with the soup.