01 - Combine sliced beef with soy sauce, sesame oil, sugar, and minced garlic. Mix thoroughly and let marinate for at least 10 minutes to infuse flavors.
02 - Bring a large pot of water to 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 until smooth and well combined. Set aside for later use.
04 - Heat a wok or large skillet over medium-high heat. Add oil and sauté carrot, bell pepper, and mushrooms for 3-4 minutes until tender-crisp. Add spinach and spring onions, cooking 1-2 minutes longer until wilted. Transfer vegetables to a plate.
05 - In the same wok, add marinated beef and stir-fry for 2-3 minutes until browned and completely cooked through.
06 - Return cooked vegetables and noodles to the wok with the beef. Pour sauce over and toss everything together for 2-3 minutes until heated through and evenly coated.
07 - Divide noodles among serving bowls. Garnish with toasted sesame seeds and cilantro or parsley if desired.