1.Place noodles in a large heatproof bowl; cover with boiling water. Stand for 4 minutes or until noodles are tender; drain.
2.Meanwhile, heat half the oil in a medium frying pan over medium-high heat; cook garlic and mushrooms, stirring, for 2 minutes or until mushrooms are tender. Season to taste. Transfer to a small bowl.
3.Lightly whisk eggs and ½ cup of the water in a large jug. Blend sauce, cornflour and remaining water in a small bowl until smooth.
4.Heat 1 teaspoon of the remaining oil in a medium frying pan over high heat; pour in a quarter of the egg mixture. Working quickly, sprinkle omelette with a quarter each of the onion, mushrooms and noodles.
5.Using a large metal spatula, gently lift edges of omelette away from pan, swirl pan to move uncooked egg to the edge of the pan. Lift the edges of the omelette away from the pan, gently moving spatula under omelette. Fold omelette in half. Pour a quarter of the oyster sauce mixture into the pan; cook for a further 1 minute or until sauce thickens. Remove omelette from pan. Rinse and wipe out pan.
6.Repeat with remaining ingredients to make four omelettes. Top omelettes with bean sprouts, chilli and coriander to serve.