Step 1: Marinate the Beef

1. Combine Ingredients: In a bowl, mix the sliced beef with soy sauce, oyster sauce (if using), and cornstarch. Let it marinate for about 10 minutes while you prepare the vegetables.

Step 2: Cook the Beef

1. Heat Oil: In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat.
2. Stir-Fry Beef: Add the marinated beef to the skillet in a single layer. Cook for about 2-3 minutes until browned and cooked through. Remove the beef from the skillet and set aside.

Step 3: Stir-Fry the Vegetables

1. Add More Oil: In the same skillet, add the remaining 1 tablespoon of vegetable oil.
2. Sauté Garlic and Ginger: Add the minced garlic and ginger, stir-frying for about 30 seconds until fragrant.
3. Add Zucchini and Bell Pepper: Add the sliced zucchini and bell pepper. Stir-fry for about 4-5 minutes until the vegetables are tender but still crisp.

Step 4: Combine and Serve

1. Return Beef: Return the cooked beef to the skillet, mixing it with the vegetables. Stir to combine and heat through for another 1-2 minutes.
2. Season: Taste and adjust seasoning with salt and pepper as needed.
3. Garnish and Serve: Serve hot, garnished with sesame seeds and sliced green onions.