Marinate the Beef
In a bowl, combine sliced beef with soy sauce, cornstarch, and sesame oil. Let marinate for at least 10–15 minutes.
Make the Sauce
Whisk together all sauce ingredients in a small bowl. Set aside.
Stir-Fry Time
Heat oil in a large skillet or wok over medium-high heat.
Sear beef in a single layer, 1–2 minutes per side, until browned but not cooked through. Remove and set aside.
Add bell peppers, onion, garlic, and ginger. Stir-fry 2–3 minutes until just tender.
Return beef to the pan. Pour in the sauce and sprinkle with black pepper.
Cook another 2–3 minutes, stirring, until the sauce thickens and coats everything.