Soak the dried mushrooms in the warm water for 15 minutes then squeeze the water out, but keep the soaking water.
Slice the fresh mushrooms.
Heat the oil a pan, fry the mushroom for 2-3 minutes, then add the sake and soy sauce. When the mushrooms are soft, add the soaking water and dashi.
Bring to the boil, and stir in the cornstarch mixture.
Once again, bring to the boil, then gently stir in the egg, making a swirl pattern.
Turn off the heat. When the eggs rise to the surface, add the vinegar to the soup. Taste, i season with salt, if needed.
Slice the spring onion diagonally, and scatter these on top of the soup, just before serving.