Cut the beef, across its grain, into long strips, about ⅓ inch/8mm thick. Put them into a bowl. Add light soy sauce, cornstarch, baking soda, salt, and water.
Mix and rub with your hand until the liquid is fully absorbed by the meat. Pour in oil and stir to coat evenly. Leave to marinate for 15 mins.
Mix the sauce
In a small bowl, mix oyster sauce, Shaoxing rice wine, light soy sauce, dark soy sauce, cornstarch, sugar, ground white pepper, and water. Then add sesame oil. Set aside.
Sear the beef
Heat 2 tablespoons of oil in a wok/skillet over medium-high heat. Put in the marinated beef. Spread out the strips into a single layer. Do not move them until the part touching the surface browns lightly.
Then stir and toss for about 30 seconds until there is little pinkness that can be seen on the surface (do not overcook). Transfer the beef to a plate leaving any oil in the wok.
Stir-fry the vegetables
Pour in the remaining ½ tablespoon of oil. Add onion and garlic. Fry over high heat until the garlic browns on the edge.
Put in bell pepper. Toss the vegetables for about 30 seconds until they’re well heated but remain crunchy.
Combine the dish
Return the beef to the wok. Stir-fry everything for a further 30 seconds or so until the beef is cooked through (no pinkness inside the strips).
Turn the heat down to low (see note 1). Stir the sauce very well then pour it over the meat and vegetables.
Give everything a final toss. Dish out when the sauce becomes thick enough to coat the spatula (see note 2).
Notes
1. if you’re using an electric stove that doesn’t change the heat level immediately, you’ll need to remove the wok away from the burner before pouring the sauce in.2. The sauce thickens pretty quickly so you need to be attentive.