In a mixing bowl, whisk together soy sauce, oyster sauce, and cornstarch until fully combined. Add the thinly sliced beef, ensuring each piece is well-coated in the marinade, and let it sit for at least 15 minutes at room temperature.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat until shimmering. Carefully add the marinated beef in a single layer without crowding the pan. Sear for 1-2 minutes on each side, then remove it from the skillet.

In the same skillet, add another tablespoon of vegetable oil and toss in the sliced bell peppers and onions. Sauté for 3-4 minutes until tender but still vibrant.

Stir in minced garlic and freshly grated ginger. Sauté for an additional minute, avoiding burning.

Return the cooked beef alongside any accumulated juices, then pour in beef broth or water. Stir and simmer for 2-3 minutes until the sauce thickens.

Adjust seasoning with salt and pepper as needed. Serve hot over rice or noodles.