01 - Combine sliced beef with soy sauce, sesame oil, sugar, and minced garlic in a bowl. Mix thoroughly and let marinate for at least 10 minutes.
02 - Bring a large pot of water to a boil. Add noodles and cook according to package instructions until tender. Drain, rinse under cold water, and set aside.
03 - Whisk together soy sauce, brown sugar, gochujang, rice vinegar, sesame oil, water, and grated ginger in a small bowl. Set aside.
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 tender. Add spinach and spring onions, cook 1–2 minutes until wilted. Remove to a plate.
05 - In the same wok, add marinated beef and stir-fry for 2–3 minutes until browned and cooked through.
06 - Return cooked vegetables and noodles to the wok with beef. Pour in sauce and toss everything together for 2–3 minutes until heated through and evenly coated.
07 - Divide noodles among bowls. Garnish with toasted sesame seeds and cilantro or parsley if desired.