Hot and sour soup (suan la tang) with tofu and wood ear mushrooms

Method

1. Boil 650 g water in the kettle. Put the shiitake and wood ear mushrooms in a bowl, cover with 250 g of the boiling water, press down to submerge and soak for 15 minutes; keep the remaining 400 g water hot. Meanwhile, cut the tofu into batons 5 mm thick and 4 cm long, cut the bamboo shoots and carrot into fine matchsticks, slice the spring onions, keeping whites and greens separate, and peel the ginger, cut into 3 thin slices and crush with the flat of a knife. Beat the egg in a bowl and mix the cornflour with 35 g cold water.
2. Lift the mushrooms from the bowl by hand, squeezing them over it. Leave the soaking water to settle for 1 minute, then slowly pour it through a fine sieve into a saucepan, leaving the last centimetre of sandy liquid in the bowl. Remove the small tough stem from the wood ear mushroom and cut into fine strips; if the shiitake slices are wide, cut them into strips too.
3. Add the 400 g hot water, crumbled stock cube, ginger, spring onion whites, both types of mushroom, bamboo shoots and carrot to the saucepan. Bring to a simmer, about 2 minutes, then simmer over a medium heat for 10 minutes. The broth should smell earthy and the wood ear mushroom should stay crisp.
4. Remove the ginger slices. Add both soya sauces and the sugar; the broth takes on its amber colour. Slip in the tofu without stirring vigorously, to avoid breaking the batons, and simmer for 3 minutes. Taste: the broth should be distinctly salty, since the vinegar and tofu soften the seasoning; if needed, add 1 tsp light soya sauce. Stir the cornflour mixture in its bowl, pour it in a thin stream while stirring gently and cook for 1–2 minutes, until the broth is glossy, translucent and coats a spoon generously.
5. Maintain a gentle simmer. Stir the soup once in a circular motion, pour in the beaten egg in a thin stream, wait 20 seconds, then stir gently. Cook for another minute, until fine ribbons of fully set egg form and remain suspended in the thickened broth.
6. Turn off the heat. Add 20 g black vinegar, the white pepper and sesame oil, and stir once. Divide between two warm bowls, add 5 g vinegar to each and scatter over the spring onion greens. Serve immediately, with a clear acidity and the definite heat of white pepper at the back of the throat.