Heat a wok or large frying pan over a medium heat. Add the oil and, when the oil is hot enough, add the leeks and stir-fry for 3 minutes.
Mix the cornflour and salt together in a large bowl. Add the prawns to the cornflour and salt mixture, and toss to coat all over.
Add the prawns to the wok, and stir-fry for 2 minutes or until the prawns have changed colour and are almost cooked through.
Now add the mushrooms and beansprouts to the wok, and stir-fry for a further 2 minutes.
Beat the eggs with 3 tablespoons water in a small bowl. Pour the egg mixture into the wok and cook, stirring, until the egg sets, carefully turning the omelette over once.
Turn the omelette out onto a clean board, divide into four and serve hot.