Heat the oil in a wok or large frying pan over high heat. Add the oil and the onion and stir-fry for half a minute or until fragrant. Add the chicken and stir-fry until almost cooked. Add the capsicum and stir-fry for another minute. Then add the noodles and the marinade. Stir-fry until well combined. Remove from heat. Top with spring onion and serve.