To make the sauce, drain the pineapple in a sieve over a bowl and keep all the juice (you should have about 150ml) Put the corn flour in a large bowl and stir in three tablespoons of the pineapple juice to make a smooth paste. Add the remaining juice and 150ml water, then stir in the soy sauce, vinegar, sugar, ketchup and chilli flakes until thoroughly combined. Set aside. For the rice, beat together the egg and sesame oil and put to one side. Heat the vegetable oil in a wok or large frying pan. When it’s shimmering and almost smoking, add the rice and stir-fry for about 3-4 minutes until completely heated through. Add the peas, spring onions and bean sprouts, if using. Stir-fry, turning the rice constantly around the pan, for about 3 minutes. Season well with soy sauce and pepper, then push to one side of the pan. Pour the beaten egg mixture into the other side and leave for about 10 seconds so it begins to set. Using a chopstick, briskly swirl around the egg to break it up and then toss around with the rice. Stir-fry for a further minute and serve straight away with the sweet and sour chicken.