For the sauce: Combine all the ingredients in a small bowl. Set aside.
Toss the sliced chicken thighs with the cornflour until evenly coated.
Heat the oil in a large frying pan or wok over medium–high heat. Add the chicken and stir-fry until cooked through and golden brown, about 5–7 minutes. Add a little of the sauce in the last minute and stir through. Remove the chicken and set aside.
In the same pan, add a little more oil if needed. Sauté the onion for about 2–3 minutes. Add the bok choy and broccolini and stir-fry until the vegetables are tender-crisp, about 2 minutes. Then toss through the garlic and ginger. Pour the remaining sauce over the vegetables. Toss everything quickly.
Return the chicken to the pan, add the cashews and stir-fry for another minute to heat through. Serve on white or brown rice.