01 - Combine beef with cornstarch, 1 tablespoon soy sauce, and 1 teaspoon sesame oil in a bowl. Let marinate for 10 minutes.
02 - Prepare noodles following package directions. Drain and set aside.
03 - In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, black pepper, and water. Set aside.
04 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Add marinated beef and stir-fry for 2 to 3 minutes until browned. Remove and set aside.
05 - Add remaining vegetable oil to the wok. Stir-fry garlic, broccoli, and carrot if using for 3 to 4 minutes until broccoli is bright green and crisp-tender.
06 - Return beef to wok. Add noodles and sauce, tossing for 2 to 3 minutes until heated through and evenly coated.
07 - Stir in sliced spring onions. Sprinkle with toasted sesame seeds if desired. Serve immediately.