In a small bowl add water and cornstarch and stir until cornstarch is fully dissolved into the water. Set aside.
In a large microwave-safe bowl or mug that can hold at least 20 oz of liquid, add broth, soy sauce, rice vinegar, and chili oil. Stir to mix. Taste and adjust as needed.
Add in tofu and mushrooms. Stir them into the broth.
Place a paper towel over mug and put mug into microwave. Cook at full power for 2 minutes. Remove briefly and stir. Place back into microwave and cook for an additional 1 minute or until mushrooms are cooked.
While stirring the hot soup, add in the cornstarch slurry (if the cornstarch has separated from the water, give it a quick stir before adding it in) Make sure to do this while stirring the soup, otherwise the cornstarch will clump up. Stir until the cornstarch is fully mixed in.
Drizzle in the egg over the top and sprinkle ground pepper (you do not need to be stirring the soup as you do this). Use a fork to stir a few times, just until you see the egg start to ribbon. The soup should be hot enough to partially cook the egg so you will see the ribboning.
Cook the soup an additional 1 minute or until thickened and egg is fully cooked. Garnish with scallions.
Notes
I poured the soup into a smaller mug after it finished cooking, for photo purposes. But make sure you use a mug or bowl that can hold at least 20 oz of liquid to prevent the liquid from overflowing while it cooks.
I use an 1100 watt microwave. Cooking time may need to be adjusted if your microwave wattage is different.
There is a range of sodium levels in low sodium soup so be sure to adjust sauces as needed.
Hot & sour soup is traditionally made with shitake mushrooms, but some people find the flavor too strong so I used crimini mushrooms for this version. If you like shitake mushrooms, you can use that instead.
Rice vinegar is not the same as distilled white vinegar.
You can add more chili oil if you enjoy a spicier soup. You can also use srircha sauce if you don't have chili oil.
Make sure to fully whisk an egg in a bowl before measuring out 1 1/2 tbsp of the whisked egg for the soup.
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.