Heat a wok or large frying pan until it is very hot, then add 1 tbsp of the oil. When it is very hot, add the chicken and stir-fry over a high heat for 8-10 minutes, until browned all over. Using a slotted spoon, take the chicken from the pan and set aside.
Reheat the wok and add the remaining oil. Toss in the garlic and onions and stir fry for 3 minutes, until golden brown.
Return the chicken to the wok and add the red pepper, chillies, fish sauce, dark soy sauce and sugar. Stir fry over a high heat for a further 8-10 minutes or until the chicken is cooked through. Stir in the basil leaves and serve with rice garnish with fresh basil.