Heat the oil in a wok or large frying pan over a medium heat. Tip in the chicken pieces and cook for 3–4 minutes, stirring frequently, until golden brown all over. Remove from the heat with a slotted spoon and set aside.
Add the onion to the wok and stir-fry for 2–3 minutes until golden and beginning to soften, then add the garlic and ginger and stir-fry for 1–2 minutes. Stir in the pineapple and sweet chilli sauce, then bring to a boil.
Return the chicken to the pan with the water chestnuts, soy sauce and lime juice and stir to combine. Reduce the heat and simmer gently for 4–6 minutes until the chicken is cooked through, then add the peas and stir for 1–2 minutes until hot. Scatter over the cashew nuts and serve immediately with steamed rice.