In a large bowl, combine the beef with the bicarbonate of soda, light and dark soy sauces, Shaoxing wine, cornflour, sesame oil and pepper. Toss well to coat and set aside for 15–20 minutes while you prepare the remaining ingredients.
In a small bowl, whisk together the oyster sauce, dark soy sauce, sugar, black pepper, beef stock and cornflour/water mix until smooth.
Heat a large deep heavy-based pan over high heat. Add 1 tablespoon of the oil and cook half the beef for 1–2 minutes until browned. Transfer to a plate and repeat with the remaining beef and oil.
To the same pan, add the onion and garlic. Cook, stirring for 1 minute until fragrant. Add the choy sum and toss for another 1–2 minutes until just wilted.
Return the beef to the pan. Add the noodles. Pour in the black pepper sauce and toss using tongs for 2–3 minutes until glossy and heated through.
Divide among four bowls and finish with extra cracked black pepper, if using.
