Prep the Beef & Marinate: Pat the thinly sliced flank steak dry using a paper towel. In a medium bowl, combine the beef with the marinade ingredients: soy sauce, rice wine vinegar, sesame oil, sugar, and cornstarch. Toss it well until the meat is coated and let it sit at room temperature while you prepare the vegetables (about 10 minutes).

Prepare the Sauce: In a small bowl, whisk together all the sauce ingredients except the cornstarch slurry: beef broth, soy sauce, oyster sauce, hoisin sauce, and sesame oil. Set aside. Also, prepare your cornstarch slurry (1 tsp cornstarch in 1 tbsp cold water) and keep it next to the stove.

Sear the Steak: Heat 1 tablespoon of oil in a large skillet or wok over high heat. Add about half of the marinated beef in a single layer (do not overcrowd the pan). Cook for 1 to 2 minutes per side until browned. Remove and set aside. Repeat with remaining oil and beef.

Sauté the Aromatics and Vegetables: Reduce heat to medium-high. Add the sliced onions and bell peppers to the skillet. Stir fry for about 3 minutes until crisp-tender. Add ginger and garlic in the final minute, stirring until fragrant.

Combine and Finish the Sauce: Return all seared beef to the pan with vegetables. Pour the sauce over and bring to a quick boil.

Thicken the Glaze: Pour the cornstarch slurry into the boiling mixture, stirring constantly. Cook 30-60 seconds until thickened. Remove from heat, garnish if desired, and serve over rice.