In a small bowl, mix soy sauce, oyster sauce (if using), sesame oil, cornstarch, and water. Set aside.
Cook the Beef:
Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat.
Add the beef strips and cook 3–4 minutes until browned but still tender. Remove and set aside.
Sauté Vegetables:
In the same pan, add remaining oil if needed.
Sauté garlic for 30 seconds until fragrant, then add sliced onions and bell peppers. Cook 3–4 minutes until vegetables are crisp-tender.
Combine & Finish:
Return beef to the pan. Pour in the sauce and toss everything together.
Cook for an additional 2 minutes until sauce thickens and coats the beef and vegetables.
Season with salt and pepper as needed.
Serve:
Garnish with sliced green onions or sesame seeds if desired.
Serve hot over steamed rice or noodles.
