In a bowl, toss sliced steak with cornstarch, soy sauce, and sesame oil. Set aside for 10 minutes.
Whisk together all sauce ingredients in a small bowl and set aside.
Heat vegetable oil in a large skillet or wok over high heat.
Add steak in a single layer and sear for 2–3 minutes until browned. Remove and set aside.
In the same pan, add onions and bell peppers. Stir-fry 3–4 minutes until slightly tender but still crisp.
Add garlic and ginger. Cook 30 seconds until fragrant.
Return steak to the pan and pour in the sauce.
Stir and cook 2–3 minutes until the sauce thickens and coats everything evenly.
Serve immediately while sizzling hot.
