Toss sliced beef with salt, pepper, and cornstarch until evenly coated.
Heat oil in a large skillet or wok over high heat. Sear beef in batches until browned. Remove and set aside.
In the same pan, add peppers, onion, and mushrooms. Stir-fry 3–4 minutes until slightly softened.
Add garlic and ginger; cook 30 seconds until fragrant.
Whisk sauce ingredients together and pour into the pan.
Return beef to the skillet and toss everything together.
Simmer 2–3 minutes until the sauce thickens and coats everything beautifully.
Serve hot.
