Step 1: Toss sliced beef with cornstarch to coat. Heat oil in a wok or skillet over high heat. Cook beef in batches until crispy and browned. Remove and set aside.
Step 2: In the same pan, add garlic, onion, and bell pepper. Stir-fry until soft and slightly charred, about 3–4 minutes.
Step 3: In a bowl, whisk together soy sauce, hoisin, brown sugar, rice vinegar, sesame oil, and water. Pour sauce into the pan and simmer for 1–2 minutes.
Step 4: Return crispy beef to the pan. Toss well to coat in the sticky sauce.
Step 5: Serve hot over steamed rice or noodles. Garnish with chopped green onions if desired.
Notes
Swap in broccoli, snap peas, or mushrooms for added veggies. For heat, add crushed red pepper or sriracha to the sauce.