Mix 1 tablespoon sesame oil with the soy sauce and mirin to make a dressing. Cut the spring onions lengthways into thin, 5 cm long strips. Cut the ginger into thin strips. Cut the garlic into thin slices.
Pour 200 ml of boiling water over the mushrooms in a bowl and soak for 10 minutes. Drain, collecting 100 ml of soaking water. Remove the woody mushroom stalks and cut the hats into strips.
Cook the pasta in boiling water for 3 minutes, then drain.
Heat the remaining oil in a wok over medium heat, add the spring onions, ginger and garlic and stir for 2 minutes. Add mushrooms and pasta, heat while stirring, then pour in the sesame dressing and mushroom water. Cover the pasta with the sauce while stirring.
Sprinkle the dish with sesame seeds and garnish with mint.