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 at room temperature.
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 running 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.
04 - Heat a large wok or skillet over medium-high heat. Add a splash of oil and sauté carrot, bell pepper, and mushrooms for 3 to 4 minutes until just tender. Add spinach and spring onions, cooking for 1 to 2 minutes longer until wilted. Transfer vegetables to a plate.
05 - In the same wok, add the marinated beef and stir-fry for 2 to 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 to 3 minutes until heated through and evenly coated.
07 - Divide the noodles among serving bowls. Garnish with toasted sesame seeds and fresh cilantro or parsley if desired.