8 cups water1 (6-inch) piece kombu (kelp) 1/3 cup katsuobushi (dried bonito flakes) 1/4 cup tamari, plus more to taste 2 tablespoons mirin or dry sherry 1 teaspoon sugar 12 ounces dried udon noodles 1 head bok choy, trimmed and sliced 3 leeks, trimmed and thinly sliced 1 pound silken tofu, cut into 1-inch cubes
To make the broth, combine the water and kelp in a saucepan and let soak for 1 to 2 hours. Add the katsuobushi and bring to a boil. Boil for 4 minutes, then strain and discard the solids. Flavor the broth with the tamari, mirin, and sugar. Let it simmer while you prepare the noodles.
Bring a large pot of salted water to a boil. Add the noodles and cook according to the package instructions. Drain well and rinse with hot water.
Add the bok choy, leeks, and tofu to the broth. Heat until the bok choy is wilted, but still crisp and the leeks are tender, about 5 minutes.
Divide the noodles among four large bowls. Ladle the broth, vegetables, and tofu over the noodles, and serve immediately.
adapted from Recipes from the Root Cellar by Andrea Chesman