Marinate the beef: Thinly slice ½ lb flank steak against the grain. In a bowl, mix beef with 1 tbsp soy sauce, 1 tsp cornstarch, 1 tbsp freshly ground black pepper, and 1 tsp oil. Let sit 5-10 minutes.

Prep vegetables: While beef marinates, thinly slice ½ head green cabbage (about 4 cups), 1 onion, and 3 garlic cloves. Keep cabbage separate from onion and garlic.

Sear the beef: Heat 1 tbsp oil in a large skillet or wok over high heat. Add marinated beef in a single layer and sear 1-2 minutes per side until browned but not cooked through. Remove to a plate.

Stir-fry vegetables: In same pan, add 1 more tbsp oil if needed. Add onion and garlic, cook 2 minutes until fragrant. Add sliced cabbage and cook 4-5 minutes until wilted but still crunchy.

Combine and sauce: Return beef to pan. Add 2 tbsp soy sauce, 1 tbsp oyster sauce (or more soy sauce), 1 tsp sugar, and ½ cup water. Cook 2-3 minutes until sauce thickens slightly. Taste and add more black pepper if desired.