Slice the sweet potato into thin 5mm slices and soak in water to remove excess starch. Drain the water. Cut the root off the shimeji mushrooms and separate them into small clusters. Slice the green onion diagonally.

In a pot, heat water and add dashi powder. Bring to a boil and add the sweet potato slices. Once boiling, add the shimeji mushrooms, cover, and simmer on low heat for 2–3 minutes until tender.