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 having some fine bonito flakes remaining, you can simply use a fine-mesh strainer. Bonito dashi is now ready.
Remove the stems from shiitake mushrooms and thinly slice the caps. Trim off the base of enoki and shimeji mushrooms and separate them into smaller or individual pieces. Thinly slice green onion.
Place 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, the aburaage, and sake, and simmer them over low heat for 3 minutes. Then, turn off the heat, add salt and soy sauce, and let the flavors blend.
Serve in each bowl and top with the green onions.