I couldn't wait to dive into this big bowl of nourishment. Japanese soba noodles swim in a savory mushroom broth dense with taro root, daikon, carrot, and kabocha squash. A Japanese market is where you will find: Naga negi, which looks like an oversize green onion (you can substitute a leek); bunashimeji mushrooms are petite, brown button-topped mushrooms sold in clusters (enoki mushrooms can be used in their place); look for precooked soba noodles, in either the frozen foods section or the refrigerator case; you'll find shichimi togarashi, a Japanese spice blend, in the spice aisle.
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Place the shiitake mushrooms in a heatproof bowl and pour the boiling water over them. Place a plate on top of the mushrooms to keep them submerged and let soak for 1 hour.
Meanwhile, cook the soba noodles. Have ready a large bowl of ice water. Bring a large pot of water to a rapid boil over high heat. Add the soba but do not stir until the water has returned to a boil. Use a long wooden spoon or long chopsticks to separate the noodles, and then immediately lift out the noodles with a sieve. Transfer the noodles to the ice water, give them a stir to remove any additional starch, and as soon as they are cool, drain them thoroughly.
Remove the mushrooms from the water, rinse them to remove any grit, blot dry, and cut off and discard the stems. Cut the mushrooms into halves or quarters, depending on their size. Set aside. Strain the soaking liquid through a fine-meshed sieve into a bowl. Add the soy sauce and mirin and set aside.
In a large saucepan or soup pot, heat the sesame oil over medium heat. Add the mushrooms and sauté, stirring constantly, for 1 minute. Add the sugar and stir just until the sugar is dissolved, about 30 seconds. Add the sake and stir to deglaze the pan, scraping up any brown bits on the pan bottom.
Add the mushroom liquid, taro, daikon, carrot, naga negi, and bunashimeji mushrooms. Increase the heat to high and bring to a boil. Reduce the heat to a simmer and cook for 5 minutes. Add the squash and continue to cook until the squash is fork-tender, 5-7 more minutes.
Divide the noodles evenly among warmed soup bowls. Make sure the soup is piping hot and then ladle it over the noodles in each bowl and serve immediately. Pass the shichimi togarashi at the table.