Hot and sour soup is a Sichuan/Shanxi broth thickened with cornstarch, flavoured with rice vinegar and white pepper, and loaded with julienned pork, wood ear mushrooms, day lily flowers, bamboo shoots, silken tofu, and egg ribbons.
The balance
Sour (vinegar) and hot (white pepper) should be roughly equal and both clearly perceptible. The cornstarch thickening allows the flavour to cling. Add vinegar off-heat — it loses acidity when boiled. Adjust white pepper at the end.
Egg ribbon technique
Beat eggs with a small amount of sesame oil. When the soup is at a steady simmer, pour the egg in a thin stream in a circle while stirring the soup slowly with chopsticks. This creates long, silky ribbons rather than lumps.