Make the sauce: In a small bowl, whisk cornflour and sugar until smooth. Stir in garlic, soy sauce, oyster sauce, and chicken stock. Set aside.
Soak the noodles: Place dried noodles in a bowl and cover with boiling water. Let sit while preparing the stir-fry.
Cook the chicken: Heat oil in a wok or large frying pan. Add chicken and Chinese five spice. Stir-fry for 6–7 minutes until almost cooked.
Add the vegetables: Stir in your veg mix and cook for another 4–5 minutes until tender-crisp.
Add the sauce: Pour your prepared sauce into the pan. Stir well to coat the chicken and vegetables. Let it bubble and thicken.
Combine with noodles: Drain the noodles and add them to the pan. Toss everything together until well mixed and heated through.
Garnish and serve: Sprinkle with spring onions (if using) and serve hot.