To make the dashi stock, wipe the kombu with a damp cloth but do not rub off the white powdery substance that will become obvious as it dries.
Cut the kombu into strips. Put the kombu and 1.5litres (6cups) cold water into a saucepan and slowly bring to the boil. Quickly add 60 ml (2fl oz/¼cup) cold water to stop the boiling process. Add the bonito, return to the boil, then reduce the heat and simmer for 15 minutes.
Remove from the heat. Allow the bonito to sink to the bottom of the pan, then strain the liquid through a fine sieve. You will need about 875ml (30fl oz/3½cups) dashi stock for the soup.
Soak the wakame in cold water for 5 minutes, or until rehydrated and glossy. Drain well, then divide among four small soup bowls. Divide the tofu and the spring onion among the bowls.
Pour the dashi into a saucepan and bring to the boil. Combine the miso with 250ml (9fl oz/1cup) of the hot dashi in a bowl and whisk until smooth. Return the miso mixture to the pan and stir until combined — be careful not to boil the broth, as this will diminish the flavour of the miso.
Ladle into the bowls until they are two-thirds full and serve immediately. To drink the miso, sip the soup from the bowl — rest the bowl in your left hand and tilt to your lips with your right. Then use chopsticks to eat the solid ingredients.