Slice Shiitake thinly. Break apart Shimeji and Maitake into small bunches of mushrooms.

Cut long onion into 2" long pieces. Slice vertically into the center, but not cut through. Take out the middle part of the onion, flatten the outer layers, and slice very thinly. Put the sliced onion in the water for 10 minutes, then strain. (The middle part of the onion can be used for some other dish)

Heat Dashi at medium heat, add Soy Sauce, Sake, Mirin and salt, and heat to just before a boil. Add all the mushrooms and cook for a few minutes until they become soft. Combine water and potato starch together in a small bowl, and mix well. Add the slurry to the soup slowly while stirring constantly. Keep it warm at low heat.

Cook dried Soba according to the package. Put one serving of Soba in a bowl and pour the soup over. Top with sliced onion. Sprinkle Shichimi pepper if you like.