Mix the sauce: Whisk the soy sauce, oyster sauce, beef broth, and cornstarch in a small bowl until completely smooth.
Sear the beef: Heat one tablespoon of oil in a large skillet over medium-high heat until shimmering. Add the thinly sliced beef in a single layer, sear for two minutes per side, then remove from the pan.
Cook aromatics: Add the remaining oil to the skillet, tossing in the garlic and ginger for thirty seconds until fragrant.
Sauté vegetables: Stir in the onions and peppers, cooking for three to four minutes until they are bright but still crisp.
Combine and thicken: Return the beef to the skillet, pour the prepared sauce over the top, and let it bubble and thicken for one minute before serving.
