In a bowl, combine the beef strips, soy sauce, oyster sauce, and cornstarch. Toss until evenly coated. Let sit for 15 minutes to tenderize and flavor the meat.

Heat a large skillet or wok over medium-high heat until very hot. Add 1 tablespoon of vegetable oil and swirl to coat.

Add the minced garlic and sauté for 20 to 30 seconds until fragrant; do not let it burn.

Spread the marinated beef in a single layer. Let it sear without moving for 30 to 60 seconds, then stir and cook for about 3 to 4 minutes total until browned but not overdone. Transfer the beef to a plate and set aside.

Add the remaining 1 tablespoon oil to the pan if needed. Add the bell pepper and onion and stir-fry for 2 to 3 minutes until they are tender-crisp and slightly softened.

Return the beef to the pan and toss everything together for 30 to 60 seconds. Taste and season with salt and black pepper. If the sauce seems thin, let it bubble briefly to reduce and thicken.

Serve immediately over steamed rice or toss with hot noodles.