Put the kombu in a saucepan and pour in 1 litre of water. Bring slowly to the boil, then remove from the heat and cover the pan. Set aside for 5 minutes. Use a slotted spoon to remove and discard the kombu.
Stir the sake, rice wine or sherry, sugar and ginger into the broth and return to the boil. Reduce the heat and stir in the miso until it dissolves completely.
Add the spring onions, mushrooms, tofu and watercress. Cook very gently, stirring, for 2 minutes without allowing the soup to boil. Ladle into small bowls and serve at once.