Suan la tang (酸辣汤, hot and sour soup) is a northern Chinese preparation of pork or mushroom stock thickened with cornstarch and flavoured with Chinkiang vinegar (for the sourness) and white pepper (for the heat) — the defining flavour pairing of this soup. It contains wood-ear mushrooms, tofu strips, reconstituted lily buds, bamboo shoots, and egg ribbons (achieved by drizzling beaten egg into the hot, thickened soup while stirring rapidly). Despite its ubiquity in Western Chinese restaurants, authentic suan la tang uses white pepper rather than chilli for its heat, and Chinkiang vinegar (not white vinegar) for its sourness.
The stock: Rich pork or chicken stock — the depth of the stock is the backbone of the soup. The preparation: Reconstitute dried wood-ear mushrooms and lily buds in warm water. Slice the wood-ear mushrooms and squeeze the lily buds. Julienne the bamboo shoots. Dice firm tofu. The thickening: Cornstarch slurry (3 tbsp cornstarch + 4 tbsp cold water per 1L of stock) added to the simmering soup and stirred until the soup coats the back of a spoon lightly. The egg ribbons: Beat 2-3 eggs. Hold the beaten egg above the simmering (not boiling) soup. Pour in a thin stream while stirring the soup in a circular motion with a chopstick — the egg sets in ribbons distributed throughout the soup. The seasoning: 3 tbsp Chinkiang vinegar, 1 tsp white pepper, 2 tbsp light soy, 1 tsp sesame oil, added at the very end off heat.
Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009); Ken Hom, Complete Chinese Cookbook (2011)