In a mixing bowl, whisk together the soy sauce, oyster sauce, and cornstarch until smooth. Add the thinly sliced beef, tossing it well to coat thoroughly. Allow the beef to marinate for at least 15 minutes at room temperature.

Heat 1 tablespoon of vegetable oil in a large skillet over high heat. Once shimmering, add the marinated beef in a single layer. Sear for 1-2 minutes on each side until browned and just cooked through. Remove from the skillet and keep warm.

In the same skillet, add the remaining tablespoon of vegetable oil. Toss in the sliced red and green bell peppers and onion. Sauté for 3-4 minutes until softened but still crunchy.

Stir in the minced garlic and grated ginger into the skillet. Cook for an additional minute until fragrant, stirring constantly to prevent burning.

Return the seared beef to the skillet, combining it with the sautéed vegetables. Pour in the beef broth or water, stirring until well mixed. Bring to a gentle simmer, cooking for about 2-3 minutes until the sauce thickens slightly.

Taste and season with salt and pepper. Serve warm over steamed rice or noodles.