Add broth to a large pot. Mix in rice vinegar, soy sauce, sriracha, ginger and white pepper. Bring to a simmer. Taste and adjust as needed.
Gently slide in tofu and mushroom. Cook at a simmer until mushrooms and tofu are cooked. This should only take a few minutes. Taste soup again and adjust seasonings if needed.
Turn off heat. In a small bowl, whisk together cornstarch and water until cornstarch is fully dissolved. Pour mixture into soup and mix it right away so that it dissolves into the soup. It is important to stir it right away so that the cornstarch does not clump up.
Bring soup to a simmer again and allow it to thicken, 1-2 minutes. While gently stirring the soup, pour the whisked egg in at a steady stream, creating ribbons with the stirring. Turn off heat once the egg is cooked (this only takes a few seconds).
Garnish with scallions and serve. Soup is traditionally served with more white pepper, chili oil and black vinegar.
Notes
Shiitake mushrooms have a strong flavor that some people do not like. If that is the case, you can replace with cremini mushrooms.
American Chinese restaurants tend to make the soup extra thick. The traditional Chinese version is not quite as thick. If you like your soup extra thick, you can add 3 tbsp of cornstarch instead of 2 (dissolved in 3 tbsp of water).
Cornstarch must always be dissolved in cold water and quickly mixed into the broth before it boils. If you try to mix it in while the soup is boiling, the cornstarch will cook and clump and you will have globs of cornstarch in the soup.
Use rice vinegar. Regular white vinegar has a harsh sour taste. Rice vinegar has a more delicate sour flavor and is also sweeter.
Other ingredients you can add to the soup include wood ear mushrooms, bamboo shoots, pork, and lily buds.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.