Hot and Sour Soup

Put the dried shiitake in a heatproof bowl and pour over the 200ml of just-boiled water.

Soak for 30 minutes, until the caps are soft right through and give when you pinch the thickest part.

Lift the mushrooms out and squeeze them over the bowl.

Slice them thinly, discarding the hard stems.

Pour the soaking liquid through a fine sieve into a jug and stop before the last spoonful. Grit settles at the bottom, and the liquor above it is worth more than the mushrooms themselves.

Heat the oil in a large pan over medium-high heat.

Fry the ginger for 30 seconds, until it smells sharp.

Add the fresh mushrooms and cook for 4-5 minutes, until they have given up their water and begun to brown at the edges.

Pour in the stock and the strained mushroom liquor.

Add the sliced shiitake, the bamboo shoots, the soy sauce, the sugar and the white pepper.

Simmer for 8-10 minutes, until the broth tastes savoury rather than like seasoned water.

Slide in the tofu and warm it through for 2 minutes, stirring gently and as little as you can. Tofu breaks into crumbs under a hard spoon and clouds the soup.

Whisk the cornflour into the 60ml of cold water until there is not a lump left.

Whisk it again immediately before it goes in. A slurry left standing even a few minutes settles into a cake at the bottom of the cup, and what you pour in is water.

Pour the slurry into the simmering soup in a thin stream, stirring constantly.

Never tip cornflour in dry. It seizes into lumps the moment it meets hot liquid and no amount of stirring breaks them up again.

Simmer for 1-2 minutes, until the soup turns from cloudy to glossy and coats the back of a spoon.

Turn the heat down until the surface is barely shivering.

Stir the soup in one direction to set it turning.

Pour the beaten eggs in a thin stream from about 20cm above the moving surface.

Leave it alone for 10 seconds before stirring again. That pause is what sets the egg into ribbons; stirred straight away it breaks into a haze and clouds the whole pot.

Take the pan off the heat.

Stir in the black vinegar and the sesame oil.

Add the vinegar last and off the heat. Sourness here is a top note and it boils away within a minute, so a soup simmered with the vinegar in tastes dull and needs twice as much to fix.

Taste and season with fine salt and more white pepper, which is where the heat in this soup comes from rather than chilli.

Scatter the spring onions over the bowls as you serve.

Serve straight away. Cornflour keeps thickening as it cools and loosens again on reheating, so a bowl poured an hour later is neither one thing nor the other.

Cool any left over in a shallow container, refrigerate within two hours and eat it within two days, accepting that the egg will have toughened.