1 cup packed spinach leaves, washed, dried and coarsely chopped
4 scallions, finely chopped
1/4 cup grated daikon radish, (see Note)
Bring at least 3 quarts water to a boil in a large pot. Slowly add soba. When water returns to a boil, add 1/2 cup cold water. Repeat steps of returning water to a boil and adding cold water 2 or 3 times, until the noodles are just tender. (It will take 5 to 7 minutes total.) Drain and rinse with cold water, working your fingers through the strands to separate them. Set aside.
Combine broth and ginger in the large pot and bring to a boil. Reduce the heat to medium-low, add mushrooms and simmer for 8 minutes. Add sake (or mirin), soy sauce (or tamari) and vinegar.
Whisk a ladleful of the broth with miso in a small bowl to dissolve it; return the mixture to the pot, along with spinach. Simmer for 2 minutes more and remove from the heat. Divide the noodles among soup bowls and ladle the soup over the top. Garnish with scallions and daikon.
Notes: Mirin is a sweet, low-alcohol rice wine essential in Japanese cooking. Look for it in your supermarket with the Asian or gourmet ingredients.
Miso is fermented bean paste made from barley, rice or soy-beans. It is available in different colors; in general, the lighter the color, the more mild the flavor. Look for miso alongside the refrigerated tofu in the market. It will keep, in the refrigerator, for more than a year.
Daikon is a long, white radish; it can be found in Asian groceries and most natural-foods stores. Commercially prepared pickled daikon radish can be found in Asian markets.