Cook the noodles as per the packet instructions, less 1 minute (ideally until they are just pliable). Rinse with cold water and set aside.
Heat a large, heavy-based frying pan over medium–high heat. Add the olive oil, garlic and ginger and cook for 30 seconds.
Add the beef and cook for 2–3 minutes until browned.
Add the sweet chilli sauce, brown sugar, tamari or all-purpose soy sauce, dark soy sauce and rice wine vinegar. Cook for 2–3 minutes until the sauce has thickened and caramelised.
Add the frozen vegetables. Place the lid on, and cook for 2–3 minutes or until the vegetables have softened to your liking.
In a small bowl, combine the water and cornflour and add the mixture to the stir-fry and cook, stirring, for 1–2 minutes until it is glossy and thickened.
Add the noodles and spring onion (reserve 1 tablespoon for garnish) and cook for 1 minute or until the noodles are heated through.
Serve topped with the reserved spring onion and chilli oil, if using.