1

In a large (5-quart) 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.

2

In a small bowl, whisk together 3 tablespoons of the vinegar and the cornstarch. Add to the pot; simmer, stirring, until the soup is thickened, about l minute.

3

Add the egg through a slotted spoon, and stir to form ribbons. Stir in the tofu. Remove from the heat; let stand, covered, for l minute. Put the ginger in a small sieve, and squeeze to release its juice into the soup (discard the solids). Taste; add the remaining tablespoon vinegar, if desired. Serve sprinkled with the scallions.