01 - Combine the sliced beef with soy sauce, sesame oil, sugar, and minced garlic in a bowl. Mix thoroughly and let marinate for at least 10 minutes to absorb flavors.
02 - Bring a large pot of water to a rolling boil. Add noodles and cook according to package directions until just tender. Drain well, rinse under cold water to stop cooking, and set aside.
03 - Whisk together soy sauce, brown sugar, gochujang, rice vinegar, sesame oil, water, and grated ginger in a small bowl until sugar dissolves completely. Set aside for later use.
04 - Heat a large wok or skillet over medium-high heat. Add oil, then sauté carrot, bell pepper, and mushrooms for 3–4 minutes until just tender. Add spinach and spring onions, cooking 1–2 minutes more until wilted. Transfer vegetables to a plate.
05 - In the same wok, add the marinated beef and stir-fry for 2–3 minutes until browned and cooked through, stirring frequently to prevent sticking.
06 - Return cooked vegetables and noodles to the wok with the beef. Pour in the sauce and toss everything together for 2–3 minutes until heated through and evenly coated with sauce.
07 - Divide the noodles among serving bowls. Garnish with toasted sesame seeds and cilantro or parsley if desired. Serve immediately while hot.