In a small bowl, whisk the oyster sauce, soy sauce, chilli garlic sauce, if using, and 2 tbs water.
In a large pot of heavily salted boiling water, cook the noodles for 4-5 mins or until al dente. Rinse the noodles under cold water. Drain.
Heat 1 tbs of the oil in a wok or large heavy frying pan over medium-high heat. Add the chicken and cook, without stirring, for 2 mins or until the chicken is well browned on the bottom. Continue cooking, breaking up the chicken into small clusters, for 2-3 mins or until just cooked through. Transfer the chicken to a heatproof bowl and set aside.
In the wok over high heat, heat the remaining 1 tbs oil. When the oil is smoking, add the mushrooms, carrot and onion and stir-fry for 2 mins or until the mushrooms are tender. Stir in the cooked chicken and then the sauce mixture. Add the noodles and toss until well combined and heated through.
Divide the noodle mixture among 4 serving bowls. Garnish with spring onion and serve immediately.