Toss the beef with the oyster sauce, soy sauce, cornflour, baking soda and white pepper. Let it sit for 15 minutes. Whisk together all the sauce ingredients.
Prepare the noodles according to the packet directions, then drain and set aside.
Heat a large wok or frying pan with a little oil over high heat. Cook the beef in batches, in a single layer for 1–2 minutes per side until browned. Remove.
Cook the onion for 2 minutes. Stir in the garlic, ginger and dried chili peppers for 30 seconds until fragrant.
Add the capsicums and mushrooms and cook for another 3 minutes until softened.
Add the noodles to the pan and pour in the sauce and beef. Simmer for 2 minutes, until the sauce becomes glossy and lightly thickened. Stir through the spring onions before serving.
Garnish with sesame seeds, extra spring onion and chili flakes if desired.
