Coat the pork: Toss the pork matchsticks with the 1 tablespoon of water until it's absorbed, then with the vegetable oil and the 1 tablespoon of cornstarch. That thin coating keeps the pork tender in the broth instead of tightening into little strings.

Cook the pork: Bring the chicken broth to a boil in a large Dutch oven. Add the pork, stirring right away to separate the pieces, and simmer 2-3 minutes until no pink remains. Skim off any gray foam. Pork loin is a whole-muscle cut and USDA calls for 145°F, but matchsticks this thin pass that within a minute in boiling broth and are far too thin to probe accurately, so go by the pink disappearing. The soup keeps simmering well past that point.

Simmer the vegetables: Add the shiitake and bamboo shoots and simmer 5 minutes, until the mushrooms are soft and have shrunk by about half.

Season the broth: Stir in the soy sauce, the dark soy sauce if you're using it, the sugar, and the sesame oil.

Thicken it: Whisk the 1/3 cup of cornstarch into the 1/3 cup of cold water until there are no lumps. Stir the soup in a slow circle and drizzle in about three quarters of the slurry. Simmer 1-2 minutes, until the broth turns glossy and coats a spoon. Add the rest if you want it thicker.

Add the tofu and egg ribbons: Stir in the tofu. Turn the heat to low, stir the soup in a slow circle, and pour the beaten eggs in a thin stream around the pot. Stop stirring for 15 seconds and let the eggs set into ribbons, then give it one gentle stir.

Finish off the heat: Take the pot off the burner. Stir in the rice vinegar and white pepper. Taste it, then add more vinegar for sour or more white pepper for hot, a little at a time. Ladle into bowls and top with the scallions.