Freeze the steak for 15 minutes to firm it up. Slice thinly against the grain.
Toss with soy sauce, cornstarch, sesame oil, and black pepper. Set aside while you prep the vegetables.
Whisk soy sauce, oyster sauce, rice vinegar, brown sugar, beef broth, cornstarch, and chili-garlic sauce in a small bowl. Keep it near the stove.
Heat 1 tbsp oil in a wok or large skillet over high heat until shimmering.
Spread half the steak in a single layer. Let it sear 45–60 seconds before stirring. Cook 1–2 minutes total until just browned. Transfer to a plate.
Repeat with remaining oil and steak. Do not crowd the pan.
Add the onion and bell peppers to the hot pan. Stir-fry 2–3 minutes until crisp-tender with a little char.
Add garlic and ginger. Cook 30 seconds until fragrant.
Return steak (and any juices) to the pan.
Whisk the sauce and pour it in. Stir constantly 30–60 seconds until the sauce thickens and coats everything.
Taste and adjust with a splash of soy for salt, vinegar for brightness, or sugar for balance. Finish with black pepper.
Serve immediately over hot rice. Garnish with scallions and sesame seeds.
