Ken Hom's Chinese-style Chicken and Broccoli

Separate the broccoli heads into small florets and peel and slice the stems. Blanch the broccoli pieces in a large pan of boiling salted water for several minutes, then immerse them in cold water. Drain thoroughly.
Cut the chicken into 2.5-cm (1-in) pieces and combine them with the pepper, fish sauce and light soy sauce. Mix well and allow to marinate for 30 minutes. Dust the chicken pieces with cornflour, shaking off any excess.
Heat a wok or large frying-pan over a high heat until it is hot. Add the 400 ml (14 fl oz) of oil and, when it is very hot and slightly smoking, deep-fry the chicken pieces for 8 minutes or until they are golden brown. Remove them with a slotted spoon and drain on kitchen paper. You may have to do this in two or more batches.
Drain off all the oil. Re-heat the wok and add the 1 tablespoon of oil. When it is hot, add the garlic and onion and stir-fry for 1 minute. Then add the blanched broccoli and continue to stir-fry for 1 minute. Add the Shaoxing rice wine or dry sherry and water or stock and continue to stir-fry at a moderate to a high heat for 4 minutes until the broccoli is thoroughly heated through. Add the tomatoes, oyster sauce, fish sauce, sugar, salt, pepper and sesame oil and continue to stir-fry for 30 seconds, then add the drained chicken and stir-fry for 2 minutes or until the chicken is thoroughly heated through. Transfer to a warm platter and serve at once.