Marinate the chicken pieces in dry sherry for half an hour. Reserve some spring onion greens for garnish. Mix two tablespoons cornflour in half a cup of water.
Mix soy sauce, oyster sauce, white pepper powder, sugar and salt in chicken stock. Add remaining cornflour to the marinated chicken and mix well.
Heat sufficient oil in a wok and deep fry chicken pieces for two minutes or until light brown in colour. Drain on absorbent paper. Heat one tablespoon oil in a wok or a pan, add garlic and stir fry for half a minute.
Add spring onion and continue to stir fry for a minute more. Add shredded spinach and cook for two minutes on high heat, stirring and tossing continuously. Remove and arrange the cooked spinach on a serving platter and keep warm.
Heat remaining oil in a wok, add Sichuan peppers and immediately stir in the sauce and spice mix. Add fried chicken pieces and cook for two to three minutes.
Stir in the cornflour mixture, white vinegar and cook for a minute or until the sauce thickens and coats the chicken pieces. Place the chicken on the platter with the cooked spinach and sprinkle red chilli flakes.
Serve hot, garnished with the reserved spring onion greens.