Place the flour in a small bowl and season with the salt and pepper.
Roll and coat the chicken pieces in the flour mixture.
Heat the vegetable oil in a suitably sized frying pan or wok over a medium heat. Add the coated chicken and stir-fry until no longer pink inside, about 6 – 7 minutes. Then remove chicken from the pan using a slotted spoon and set aside.
Add the garlic, onion and pepper and stir-fry for about 3 – 4 minutes.
Return the chicken to the pan and stir in the sugar, vinegar, tomato puree, soy sauce, water and sherry. Allow the liquid to simmer for a couple of minutes in the pan before serving.