Strips of tender beef stir-fried with colorful bell peppers in a savory sauce make a quick, fresh dinner. This high protein dish is a family-friendly classic with bold, satisfying flavor.
In a medium bowl, toss the thinly sliced beef with 1 tablespoon soy sauce and 1 tablespoon cornstarch until well coated; let it rest while you prepare the vegetables.
Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat. Add beef in a single layer and sear for 1–2 minutes per side until just browned but not fully cooked through. Remove beef and set aside.
Add remaining 1 tablespoon oil to the same pan. Stir-fry the sliced onion and minced garlic over medium-high heat for 1 minute until fragrant but not browned.
Add bell pepper strips and cook, stirring frequently, for 2–3 minutes; keep the heat high to maintain some crunch and bright color.
Return beef to the pan. Add remaining soy sauce, oyster sauce, cornstarch, and black pepper. Toss rapidly for 2–3 minutes, making sure the sauce thickens and coats everything. Remove from heat immediately to avoid overcooking the beef. Serve hot.