Bring the bonito stock to a simmer. Add the tofu and mushrooms and heat through 3 to 4 minutes. Add the miso and mix and stir to dissolve completely. Remove from the heat, ladle into bowls, sprinkle with green onions and serve.
Note: Bonito flavoured soup base (dried flakes of a type tuna sold in powdered form), sometimes called dashi, is available in the Asian foods aisle. If it is unavailable at your store, try making the soup with vegetable or chicken stock.