Ten years ago, this dish would have been impossible to make outside Japan, as the only mushrooms we could lay our hands on were fresh champignons or dried shiitake. Actually, it would still have worked, but it’s so much better this way.
Cook noodles in boiling water for 1 minute, rinse under cold water, drain and reserve. Soak wakame in lukewarm water and leave to swell for around 15 minutes. Drain tofu and cut into 1.5 cm (½ in) cubes.
Bring dashi to the boil in a saucepan. Whisk 2tablespoons hot dashi into the miso paste