In a bowl, mix the sherry and cornflour to a smooth liquid. Add the soy and hoisin sauces, sugar and stock. Set aside.
Heat the oil in a wok or large frying pan, toss in the garlic and whites of the spring onions and stir fry for 2-3 minutes. Tip in the chicken and stir fry over a high heat until it colours, using the back of a spoon to break up any big lumps. Tip in the water chestnuts and stir fry for a further 1-2 minutes.
Push the chicken mixture to one side of the wok. Pour the sherry mixture into the empty part and stir for 1-2 minutes until it bubbles and thickens. Combine thoroughly, then leave to simmer for 5-10 minutes. Season and stir in the green spring onions and coriander.
Lay the lettuce leaves on serving plates, spoon in the warm mixture without overfilling, and scatter with the shredded spring onion. Suggest your guests roll the lettuce up around the filling to make a shape that’s easy to eat as finger food.
Prepare ahead; you can cook the filling the day before, leave to cool and store in a plastic container in the fridge overnight. Reheat it well for 5-10 minutes until hot, stirring all the time and adding a splash of hot water if it’s too dry.