01 - Combine sliced beef with 1 tbsp soy sauce, 1 tsp sesame oil, sugar, and minced garlic in a bowl. Mix thoroughly to coat evenly. Let marinate at room temperature for 10 minutes to tenderize and infuse flavors.
02 - Bring a large pot of water to a rolling boil. Add noodles and cook according to package directions until just tender (al dente). Drain thoroughly, rinse under cold running water to stop cooking and prevent sticking. Set aside.
03 - Whisk together 3 tbsp soy sauce, brown sugar, gochujang, rice vinegar, 2 tsp sesame oil, water, and grated ginger in a small bowl until sugar dissolves completely. The sauce should be smooth and well-combined.
04 - Heat a wok or large skillet over medium-high heat. Add 1 tbsp oil, then sauté carrot, bell pepper, and mushrooms for 3-4 minutes until slightly tender but still crisp. Add spinach and spring onions, cook 1-2 minutes more until spinach wilts. Transfer vegetables to a plate.
05 - Return wok to high heat. Add marinated beef in a single layer and stir-fry for 2-3 minutes, breaking apart pieces, until browned and cooked through. Do not overcrowd the pan—cook in batches if necessary.
06 - Return cooked vegetables and noodles to the wok with beef. Pour prepared sauce over everything. Toss vigorously with tongs for 2-3 minutes until all components are heated through and evenly coated with sauce. The noodles should glisten slightly.
07 - Divide noodle mixture evenly among 4 bowls. Sprinkle toasted sesame seeds over the top. Add fresh cilantro or parsley if desired. Serve immediately while hot.