Heat a wok until hot, add half the oil and swirl to coat.
Add the onion and stir-fry for 2-3 minutes or until softened.
Remove from the wok and set aside.
Reheat the wok, add another teaspoon of oil and half the butter, and stir-fry the chicken in two batches for 3-5 minutes each batch, or until browned, adding the remaining oil and butter between batches.
Return all the chicken to the wok with the onion.
Stir in the rind, capers and lemon juice.
Toss well and cook until heated through.
Add the basil and season with salt and freshly ground black pepper.