Heat the oil in a medium soup pot. Sauté onions for 5 to 7 minutes or until translucent. Add the water, shiitake, kombu and crumbled 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 (use in another dish or discard). Add the shoyu. Simmer on low for 5 minutes.
Bake the mochi at 350° or pan fry for about 5 to 7 minutes or until the cubes puff up. Remove and place several pieces in each soup bowl.
Ladle the soup into bowls and garnish with scallions.
3g fat (16% calories from fat),