In a bowl, toss sliced beef with soy sauce and cornstarch. Set aside.
Heat oil in a large skillet or wok over high heat. Add beef in a single layer and sear quickly until just browned. Remove and set aside.
In the same pan, add bell peppers and onion. Stir-fry for 2–3 minutes until crisp-tender.
Add garlic and ginger and cook for 30 seconds until fragrant.
Whisk sauce ingredients together and pour into the pan. Simmer briefly until slightly thickened.
Return beef to the pan and toss everything together until coated and heated through.
Serve immediately while sizzling.