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To make the sauce, mix all the ingredients together in a small bowl until completely blended. Heat the oil in a frying pan until very hot. Throw in the chicken, cashew nuts, pepper and onion, then stir-fry for about 4-5 mins until the chicken is cooked and the nuts are toasted. Pour over the sauce and simmer with a splash of water. Remove the pan from the heat, then stir through the watercress just before serving with boiled rice.