Season the broth: Bring the chicken broth to a gentle simmer in a medium pot over medium heat, about 5 minutes. Stir in the sesame oil, soy sauce, salt, white pepper, sugar, and turmeric if you're using it. Taste it now and adjust the salt, because seasoning a thickened soup takes much longer to distribute.

Thicken it: Whisk the cornstarch into the cold water in a small bowl until there's no grit at the bottom. Pour it into the simmering broth in a thin stream while stirring, then let it simmer 1 to 2 minutes, until the soup turns glossy and lightly coats the back of a spoon.

Drop the heat: Turn the burner to low and let the soup settle to a bare simmer, around 190°F, with just a few bubbles breaking at the edge. Egg dropped into a rolling boil shreds into tough pebbles instead of ribbons.

Add the eggs: Stir the soup in a slow steady circle with a ladle and drizzle the beaten eggs in a thin stream from about 8 inches above the pot. Stop stirring the moment the eggs are in and let the pot sit undisturbed for 30 seconds so the ribbons set.

Finish and serve: Take the pot off the heat, give it one gentle stir, and ladle into bowls. Scatter the scallions over the top and serve right away, while the ribbons are still tender.