Kinoko Jiru (Japanese Mushroom Soup)

Fill a pot with water and bring it to a boil. Once boiling, reduce the heat to low, add bonito flakes, and let it simmer for 3 minutes.
Turn off the heat and strain the mixture through a sieve lined with paper towels or a cloth (such as cheesecloth). Alternatively, if you don't mind some fine bonito flakes remaining, you can simply use a fine-mesh strainer. Bonito dashi is now ready.
Remove the stems from the shiitake mushrooms and thinly slice the caps. Trim off the base of the enoki and shimeji mushrooms, and separate them into smaller stalks or individual pieces. Thinly slice the green onion.
Place the aburaage in a heatproof bowl and pour boiling water over it to remove excess oil. Remove it from the bowl, and once it has cooled enough to handle, drain off the water. Cut it in half lengthwise and slice it into thin strips.
Return the dashi to the pot and bring it to a boil. Once boiling, add all the mushrooms, aburaage, and sake, and let them simmer over low heat for 3 minutes. Then, turn off the heat, add salt and soy sauce, and let the flavors blend.
Serve in individual bowls and top with the green onions.