In a medium bowl, combine the sliced beef, soy sauce, and cornstarch. Mix well and let it marinate for about 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef in a single layer and cook for 2-3 minutes until browned. Remove the beef from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the sliced bell peppers, broccoli, and snap peas. Stir-fry for about 4-5 minutes until the vegetables are tender-crisp.
Add the minced garlic and ginger to the skillet and stir-fry for an additional 1 minute until fragrant.
Return the cooked beef to the skillet. Add the oyster sauce, sesame oil, and season with salt and pepper. Stir everything together and cook for another 2-3 minutes until heated through.
Serve the stir-fry over cooked rice or noodles.