To make the ‘dashi’ soup stock, in a large saucepan, combine the kombu and vegetable stock and heat over medium-low heat almost to the boiling point. Remove the kombu just before the mixture comes to a boil and discard.
Remove the dashi from the heat and skim any foam that rises to the surface. Let stand for 2 minutes. Strain through a fine mesh strainer into a saucepan. Keep warm over low heat.
Add the mushrooms and soy sauce to the dashi and steep until the mushrooms are tender, about 10 minutes. Remove with a slotted spoon, and, when cool enough to handle, slice thinly. Return to the liquid.
Add the miso pastes and cook over low heat, stirring to dissolve.
Add the spring onions and tofu and cook until warmed through, about 3 minutes.