Heat the oil in a wok over a high heat until the oil starts to shimmer. Stir-fry the prawns for 1 minute, then remove using a slotted spoon and set aside.
Add the mushrooms, courgette, carrot and green beans and stir-fry for 2 minutes over a high heat. Stir in the cold cooked rice and the shoyu sauce, season with pepper and mix thoroughly.
Return the prawns to the wok and stir-fry the rice mixture for a couple of minutes. Garnish with spring onion slices and serve.