01 - Combine the sliced beef with soy sauce, cornstarch, and sesame oil in a bowl. Toss to coat evenly and let marinate for 10–15 minutes while preparing other ingredients.
02 - Cook the noodles according to package instructions until al dente. Drain thoroughly, rinse with cold water to stop cooking, and set aside.
03 - Whisk together soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, and red pepper flakes in a small bowl until the sugar completely dissolves.
04 - Heat 1 tablespoon oil in a large wok or skillet over high heat. Add marinated beef in a single layer and sear for 1–2 minutes per side until browned but still tender. Remove beef from wok and set aside.
05 - Add more oil to the wok if needed. Stir-fry garlic, ginger, white parts of spring onions, bell pepper, carrot, and sugar snap peas for 2–3 minutes until vegetables are just tender-crisp.
06 - Return the beef to the wok. Add cooked noodles and pour the sauce over everything. Toss vigorously for 2–3 minutes until noodles are well coated and heated through.
07 - Remove from heat immediately. Sprinkle with sesame seeds, green parts of spring onions, and fresh coriander or cilantro. Serve hot while noodles are chewy and sauce is glossy.