In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch until smooth. Add the thinly sliced beef and coat thoroughly in the marinade. Let sit for at least 15 minutes.

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

In the same skillet, add more vegetable oil if needed, then toss in the sliced bell peppers and onions. Sauté for 3-4 minutes until they soften but remain crisp.

Stir in minced garlic and grated ginger into the sautéed vegetables. Cook for another minute until fragrant.

Return the seared beef to the skillet, pour in beef broth or water, and stir to combine. Let it simmer for about 2-3 minutes until the sauce thickens.

Taste and adjust seasoning with salt and pepper. Serve hot over rice or noodles.