Wash the pak choi thoroughly under cold running water, rinse, and drain.
Pat dry with kitchen paper.
Trim the stems from the pak choi and shred the leaves.
Heat the vegetable stock in a large heavy saucepan.
Add the pak choi and cook for 10–15 minutes.
Mix together the rice wine vinegar, soy sauce, caster sugar, and dry sherry in a small bowl.
Add this mixture to the stock, together with the sliced chilli.
Bring to the boil, reduce the heat, and cook for 2–3 minutes.
Blend the cornflour with 2 tablespoons of water to form a paste and gradually stir it into the soup.
Cook, stirring constantly, until it thickens.
Cook for a further 4–5 minutes.
Ladle the sizzling Chinese cabbage delight into warm individual serving bowls and serve immediately.