Heat the oil in a medium soup pot. Sauté the onions for 5 minutes until translucent. Add shiitake, water, kombu and bonito flakes to the pot. Cover and bring to a boil. Reduce the flame to medium-low and simmer for 25 minutes. Reduce the flame to low, remove the kombu and shiitake. Add the sea salt and shoyu. Remove the stems from the shiitake, discard stems and slice caps thin. Ladle the soup into 4 ovenproof earthenware soup bowls. Evenly sprinkle the grated mochi on top. Bake at 450° for 10 minutes or until the mochi melts like cheese. Remove, garnish with sliced shiitake, croutons and scallions before serving.
Per serving - 135 calories, 3 g fat (16% calories from fat), 5 g protein, 36 g carbohydrate, 6 g fiber, 0 mg cholesterol, 453 mg sodium