Miso soup is enjoyed in Japan throughout the day. It is served for breakfast because it is quick to prepare. At dinnertime, this light soup begins or ends the meal. Miso is a nutritious protein-rich paste made from fermented ground soybeans mixed with rice, wheat or barley. Red (aka) miso or white (shiro) miso can be used to make this soup; the former has a heartier flavor. Kombu seaweed, a form of sea kelp, is sold as dried strips in Asian markets and health-food stores. Keep the soup from boiling, which destroys its delicate flavors.
1 piece kombu seaweed, about 4 inches long
4 cups water
1 cup loosely packed dried bonito flakes
1/3 cup red or white miso
1 Tbs. mirin or sake
2 oz. fresh enoki mushrooms and/or fresh   shiitake mushrooms
1/4 lb. soft tofu, cut into 1/2-inch cubes
1 green onion, including tender green portion,   cut on the diagonal into thin slices
Using a damp kitchen towel, wipe, but do not wash, the kombu. In a saucepan over medium heat, combine the kombu and the water. Slowly bring almost to a boil, then remove and discard the kombu. Return the water to a boil and add the bonito flakes; do not stir. Immediately remove from the heat and let the flakes sink to the bottom of the pan. Pour the stock through a very fine-mesh sieve set over a large bowl. Discard the flakes.
Return 1/2 cup of the stock to the saucepan, add the miso and stir until thoroughly combined. Add the remaining stock and the mirin. If using enoki mushrooms, cut off the lower portion of the stems and discard. If using shiitakes, cut off the stems and discard. Thinly slice the caps. Bring the stock almost to a boil over medium heat. Add the mushrooms and tofu. Remove from the heat.
Ladle the soup into warmed bowls, garnish with the green onion and serve immediately.
Serves 4 to 6.
Adapted from Williams-Sonoma Lifestyles Series, Asian Flavors, by Joyce Jue (Time-Life Books, 1999).