In a mixing bowl, combine the beef strips with soy sauce and cornstarch, ensuring each piece is well-coated. Allow to marinate for at least 15 minutes.

Heat 2 tablespoons of cooking oil in a large skillet or wok over medium-high heat. Add sliced onions and sauté for about 3-4 minutes until soft.

Increase the heat to high, then add marinated beef strips. Stir-fry for 3-5 minutes until browned and tender.

Add minced garlic and ginger to the skillet, stirring rapidly for about 30 seconds. Mix in more soy sauce, oyster sauce, and water with cornstarch to thicken.

Toss in the bell pepper strips, mixing for another 2-3 minutes until well coated and heated through.