This recipe is adapted from Washoku: Recipes from the Japanese Home Kitchen by Elizabeth Andoh (Ten Speed Press 2005). If you would like to make this even heartier, add leeks and mushrooms. As written, it is a perfect starter dish.
Makes 4 servings
1 small sweet potato, about 5 ounces, cut into 1/4-inch pieces, peeled and soaked
4 cups seafood stock, preferably fresh*
1 teaspoon soy sauce
Splash of sake
4 to 6 chives, finely minced
2 tablespoons dark miso, preferably hatcho miso
1/4 cup tightly packed katsuo-bushi**
*Available at local fish markets
**Katsuo-bushi is dried, fermented and smoked fish, available in specialty Asian food stores.
Place the potato pieces and stock in the pot and bring slowly to a simmer over low heat. Skim away any froth that appears. Season with the soy sauce and sake, and continue to simmer for 3 to 4 minutes, or until the potatoes are barely tender (test with a toothpick; it should meet no resistance).
Using a slotted spoon, transfer the vegetables to individual soup bowls, dividing them evenly. Garnish each portion with the chives.
Just before serving, place the miso in a bowl with a few tablespoons of the warm broth, mixing it together to loosen the miso. Hatcho miso is especially thick, so it may take some extra effort to dissolve it. Add the miso to the stock.
Remove the pot from the heat and sprinkle in the katsuo-bushi flakes, scattering them across the surface. The fish flakes will sink as they become drenched with broth. Stir 30 to 40 seconds. Strain the soup through a coffee-filter-lined strainer or a very fine mesh strainer into the bowl holding the sweet potato pieces and chives, dividing it evenly. Discard the fish flakes. Serve the soup immediately.