For the broth:
7 cups water
1 tablespoon instant dashi
1/2 cup soy sauce
1 tablespoon sugar
1/2 pound dried soba* (buckwheat noodles)
2 carrots, sliced thin
1/2 pound spinach, coarse stems discarded and the leaves washed well, spun dry, and cut crosswise into 1 1/2-inch-wide strips
8 to 10 ounces firm tofu (preferably silken), cut into 1/2-inch cubes
3 to 4 tablespoons miso* (fermented bean paste), or to taste, if desired
2 scallions, minced
*available at Japanese markets
Make the broth: In a saucepan bring the water to a boil. Stir in the dashi and simmer the mixture, stirring occasionally, for 3 minutes. Stir in the soy sauce and the sugar and simmer the broth for 5 minutes. Strain the broth through a fine sieve into a heatproof bowl and pour it back into the pan.
In a kettle of salted boiling water cook the noodles for 3 to 5 minutes, or until they are al dente, being careful not to overcook them, drain them in a colander, and rinse them under cold water.
Add the carrots to the broth and simmer them, covered, for 5 minutes. Stir in the spinach and the tofu and simmer the soup for 1 minute. In a small bowl stir together well 1/2 cup of the soup broth and the miso and pour the mixture back into the pan. Divide the noodles among 6 large bowls, ladle the soup over them, and sprinkle each serving with some of the scallions.
