Step 1: Marinate the Beef
1. Prepare Marinade: In a bowl, combine the soy sauce, oyster sauce, cornstarch, rice vinegar, sesame oil, and black pepper.
2. Marinate Beef: Add the sliced beef to the marinade and toss to coat. Let it marinate for at least 10 minutes while you prepare the other ingredients.

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

Step 3: Stir Fry the Vegetables
1. Add More Oil: In the same pan, add the remaining tablespoon of vegetable oil.
2. Sauté Garlic and Ginger: Add the minced garlic and ginger, and sauté for about 30 seconds until fragrant.
3. Add Scallions: Add the white parts of the scallions and stir fry for another minute until they start to soften.

Step 4: Combine and Serve
1. Return Beef to Pan: Add the cooked beef back into the pan with the scallions and garlic. Stir to combine and heat through for another minute.
2. Garnish: Remove from heat and stir in the green parts of the scallions.
3. Serve: Serve hot over cooked rice or noodles.