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.
02 - Bring a large pot of water to a boil. Add the noodles and cook according to package instructions until just 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 a splash of oil, then sauté the carrot, bell pepper, and mushrooms for 3–4 minutes until just tender. Add the spinach and spring onions, cook for another 1–2 minutes until wilted. Remove 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.
06 - Return the 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.
07 - Divide the noodles among bowls. Garnish with toasted sesame seeds and cilantro or parsley if desired.