Remove the scales and cut the head off the sardines. Remove the guts, thoroughly wash with water, and wipe dry.
Remove the stems of the shimeji mushrooms and separate into small clusters. Pit the umeboshi and finely chop into a paste-like consistency. Mix the umeboshi paste with the (A) ingredients for the simmering soup.
Arrange the (1) sardines in a pan, add the ginger, the (2) soup and heat. When it starts to boil, cover with a drop-lid and turn the heat to low. If the soup does not cover the sardines, add 1/2 cup water. Occasionally baste the the sardines with the soup. Cook until the sardines are saturated with the flavor of the soup. Add the shimeji mushrooms and continue cooking. Place the sardines and the mushrooms on a dish and serve.