01 - Combine sliced beef with soy sauce, cornstarch, and sesame oil in a bowl. Toss to coat thoroughly and let marinate for 10-15 minutes at room temperature.
02 - Cook noodles according to package directions until al dente. Drain well, rinse under cold water to stop cooking and prevent sticking, then set aside.
03 - Whisk together soy sauce, oyster sauce, hoisin sauce, brown sugar, rice vinegar, and red pepper flakes in a small bowl until sugar completely dissolves.
04 - Heat 1 tbsp 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 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 just tender-crisp.
06 - Return beef to the wok. Add cooked noodles and pour in the sauce. Toss everything together for 2-3 minutes, ensuring noodles are evenly coated and beef is heated through.
07 - Remove from heat immediately. Sprinkle with sesame seeds and green parts of spring onions. Top with fresh coriander or cilantro leaves. Serve hot.