Make Dashi broth with Katsuobushi bonito flakes. Or you can use a Dashi packet according to our Dashi recipe.
Slice green onion thinly. Cut sweet potato into 1/2" (1 cm) thick rounds. Soak potato rounds in water for a few minutes.
Cook potato in Dashi for 10 minutes or until tender. Dissolve Miso paste in the soup. Add green onion just before serving.