In a medium bowl, combine the sliced beef, soy sauce, oyster sauce, and cornstarch. Mix well to coat the beef evenly. Let it marinate for about 15-20 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat.

Once the oil is hot, add the marinated beef in a single layer. Sear for about 2-3 minutes until browned, then flip and cook for another 2-3 minutes until fully cooked. Remove the beef from the skillet and set aside.

In the same skillet, add another tablespoon of vegetable oil. Add the minced garlic and ginger, sautéing for about 30 seconds until fragrant.

Add the bell peppers, broccoli, and snap peas to the skillet. Stir-fry the vegetables for about 3-5 minutes until they are tender-crisp.

Return the cooked beef to the skillet with the vegetables. Stir to combine and heat through for another 1-2 minutes.

Season with salt, pepper, and chopped green onions. Stir well to incorporate all ingredients.

Serve the beef stir fry over cooked rice or noodles.