In a mixing bowl, whisk together 2 tablespoons of soy sauce, 2 tablespoons of oyster sauce, and 1 tablespoon of cornstarch until smooth. Add the thinly sliced beef steak, ensuring each piece is thoroughly coated. Cover the bowl and let it marinate for at least 15 minutes.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Once hot, add 2 cloves of minced garlic and 1 tablespoon of minced fresh ginger. Stir-fry for about 30 seconds.

Add the marinated beef to the skillet, spreading it out into a single layer. Sear the beef undisturbed for about 1-2 minutes. Stir-fry the beef until it's browned, about 2-3 minutes more. Remove the beef from the skillet and set it aside.

In the same skillet, toss in the sliced green bell pepper, red bell pepper, and onion. Stir-fry for about 2-3 minutes until they become tender-crisp.

Return the cooked beef to the skillet with the vegetables. Sprinkle 1 teaspoon of black pepper and drizzle in 1 tablespoon of sesame oil. Mix everything together, cooking for an additional 2 minutes.

Remove from heat and sprinkle sliced green onions and optional toasted sesame seeds over the top. Serve hot with rice.