2 cans reduced-sodium chicken broth
2 T soy sauce
1/8 t red pepper flakes (I added much more)
8 oz fresh shiitake mushrooms, sliced
4 t rice vinegar
2 T cornstarch
1 large egg, beaten
1/2 package firm low-fat tofu, cut into small cubes and drained well
2 T finely grated ginger
3 scallions, thinly sliced
1 t toasted sesame oil
In a large pot, combine the broth, soy sauce, red pepper flakes and 2 cups water. Bring to a boil over medium heat. Add the mushrooms; reduce the heat, and simmer until tender, about 10 minutes.
In a small bowl, whisk together 3 T of the vinegar and the cornstarch. Add to the pot; simmer, stirring, until the soup is thickened, about 1 minute.
Add the egg through a slotted spoon, and stir to form ribbons. Stir in the tofu. remove from the heat; let stand, covered, for 1 minute. Add the ginger and the sesame oil. Taste and add the remaining tablespoon of vinegar, if desired. Serve sprinkled with the scallions.