Put dashi soup stock in a pan and bring to a boil. Cut tofu into small cubes and add them to the soup. Simmer the tofu for a few minutes on low heat. Scoop out some soup stock from the pan and dissolve miso in it. Gradually return the miso mixture in the soup. Stir the soup gently. Stop the heat and add chopped green onion. Remember not to boil the soup after you put miso in.