Toss the sliced beef with the 1 tbsp light soy sauce, cornstarch, 1 tbsp oil and the baking soda solution if using. Leave it for 15 minutes while you prep everything else.
Crack the peppercorns coarsely with the flat of a knife or a mortar. You want broken pieces, not dust. Mix the oyster sauce, dark soy, remaining light soy, sugar and water in a small bowl.
Heat a wok or large skillet over high heat until it just starts to smoke. Add 1 tbsp oil, then the beef in a single layer. Leave it for 45 seconds without touching, then stir fry for another 45 to 60 seconds until mostly browned. Tip it onto a plate. It will not be fully cooked yet.
Return the pan to high heat with the last 1 tbsp oil. Add the cracked pepper and garlic and stir for 10 seconds, until fragrant. Add the onion and peppers and stir fry for 2 minutes. They should stay a little crisp.
Pour in the sauce mix and let it bubble for 20 seconds. Return the beef and any juices to the pan and toss for 30 to 45 seconds, until the sauce coats everything and the beef is just cooked through.
Stir in the scallions, divide between two plates and serve right away with steamed rice.