In a jug, mix together the soy sauce, vinegar, orange juice, muscovado sugar and cornflour. Set aside.
Heat the oil in a wok. Add the cashews and cook for a minute until golden brown. Quickly remove the nuts and tip on to kitchen paper to drain. Add the onion slices and fry over a high heat for 3-5 minutes until browned and softened, then lift them out and add to the cashews.
Tip the chicken into the wok and stir fry for 3-4 minutes, then tip in the broccoli, mangetout and red pepper and stir fry for another 4-5 minutes until the vegetables are tender yet still crunchy.
Stir the cornflour mixture to blend the ingredients, pour into the wok and stir fry for a couple of minutes until the sauce thickens. Stir in the onion and cashews, then pile on to warm plates.