In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch, whisking until smooth. Add the sliced beef to the marinade, tossing to ensure each piece is coated evenly. Cover and let marinate for at least 15 minutes.

While beef is marinating, slice the bell peppers and onions into thin strips, mince the garlic, and grate the ginger.

Heat 1 tablespoon of vegetable oil in a large skillet over high heat. Add marinated beef in a single layer, searing for about 1-2 minutes on each side until browned. Remove beef and set aside.

In the same skillet, add remaining oil and sauté bell peppers and onions for 3-4 minutes until tender but still crisp.

Add minced garlic and grated ginger, cooking for an additional minute until fragrant. Return beef to the skillet, pour in beef broth, and simmer for 2-3 minutes until sauce thickens, seasoning with salt and pepper to taste.