Marinate the beef:
In a bowl, combine the sliced flank steak with soy sauce, cornstarch, and oil. Mix well and let sit for 10–15 minutes while you prep the other ingredients.

Prepare the sauce:
In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, sesame oil, brown sugar, water, and cornstarch slurry. Set aside.

Cook the beef:
Heat 1 tablespoon of oil in a large pan or wok over high heat. Add the beef in batches and stir-fry for 2–3 minutes until browned. Remove from the pan and set aside.

Stir-fry the vegetables:
Add the remaining oil to the pan. Add onions, bell peppers, garlic, and ginger. Stir-fry for 3–4 minutes until tender-crisp.

Combine and simmer:
Return the beef to the pan. Pour in the prepared sauce and toss everything together. Cook for 2–3 minutes until the sauce thickens and coats the meat and vegetables.

Garnish and serve:
Stir in sliced spring onions and remove from heat. Serve hot with steamed rice or noodles.

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