In a mixing bowl, combine soy sauce, oyster sauce, and cornstarch until smooth. Add the thinly sliced beef, ensuring it's fully coated in the marinade. Let it sit for at least 15 minutes.

Heat a large skillet or wok over high heat and add vegetable oil. Once shimmering, add the marinated beef in a single layer, searing for 1-2 minutes per side until browned. Set aside.

In the same skillet, add more oil if necessary and toss in bell peppers and onions. Sauté over medium-high heat for 3-4 minutes until tender yet crisp.

Stir in minced garlic and grated ginger into the sautéed vegetables, cooking for an additional minute until fragrant.

Return the seared beef to the skillet with any unused marinade and pour in beef broth or water. Stir and cook for 2-3 minutes until the sauce thickens.

Taste and season with salt and pepper as desired, removing from heat when satisfied with seasoning.