Mix the cornflour with the sherry and egg until smooth, then stir in the lemon rind and a little seasoning. Add the chicken slices and toss together, then leave to marinate for 30 minutes.
Make the sauce by mixing the cornflour in a saucepan with a little of the lemon juice until smooth. Stir in the remaining lemon juice, sherry, sugar and seasoning. Put the pan on the heat and gradually whisk in the stock. Bring to the boil, whisking until clear, thickened and smooth. Take off the heat and add the onions.
Add the oil to a wok or large frying pan, heat and, then add the chicken a few pieces at a time and cook for 4–5 minutes, turning until golden and cooked through. Lift the chicken out of the pan with a draining spoon, put it on a plate lined with kitchen paper and keep it hot. Cook the remaining chicken in the same way.
Reheat the sauce. Transfer the chicken to small bowls lined with rice, spoon a little of the sauce over the top, pour the remaining sauce into a separate bowl, then hand round so that guests can add sauce to taste.