Cook the noodles according to the packet instructions. Drain well in a colander, refresh under cold water and set aside.
Combine the stock, soy sauce and mirin in a saucepan and bring to the boil, then reduce the heat and simmer gently for 5 minutes. Add the mushrooms and simmer gently for a further 5 minutes. Add the spring onions and tofu.
Meanwhile, boil a full kettle of water. Set the noodles, still in the colander, over a sink and pour over the boiling water. Divide the noodles between serving bowls and add the soup. Serve immediately.