32 oz chicken broth, homemade or low-sodium store bought (I use Swanson 99%)
2 Tbsp reduced sodium soy sauce
1 Tbsp dark soy sauce
1/2 tsp red pepper flakes
8 oz large button mushrooms or fresh shiitakes, stems removed, caps thinly sliced
4 Tbsp rice vinegar
2 Tbsp cornstarch
1 large egg, lightly beaten
7 oz firm tofu, drained*, cut into 1/4-inch cubes
2 Tbsp finely grated peeled fresh ginger
Sriracha, to taste
3 scallions, thinly sliced
In a large stockpot, combine the broth, soy sauces, red pepper flakes, and 2 cups of water. Bring to a boil over medium heat. Add the mushrooms, reduce heat to low, and simmer, until the mushrooms are tender, 10 minutes.
In a small glass measuring cup, whisk together 3 Tbsp rice vinegar and the cornstarch. Add to the pot, and simmer, stirring, until the soup is slightly thickened, 1 minute or so.
Drizzle the egg slowly into the pot, stirring with a fork to break up the egg into shreds as it hits the hot liquid. Add the tofu, and stir. Remove the pot from heat and let it sit for a minute.
Place the ginger in a small sieve over the pot, and press down on the solids to allow the ginger juice to fall into the soup. Add the remaining tablespoon of rice vinegar, and season the soup with sriracha, to taste.
Set the soup aside, covered, for an hour or two (or more) until ready to serve. Bring the soup back up to heat. Stir in the scallions, and serve hot.