01 - Toss the sliced beef with 1 tbsp soy sauce, cornstarch, and 1 tsp sesame oil in a bowl. Let marinate while preparing remaining ingredients.
02 - Cook noodles according to package instructions. Drain thoroughly, rinse under cold water, and set aside.
03 - Whisk together 4 tbsp soy sauce, brown sugar, gochujang, mirin, 2 tsp sesame oil, garlic, ginger, and water in a small bowl until smooth.
04 - Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat. Add marinated beef and stir-fry for 2 to 3 minutes until just cooked through. Transfer to a plate.
05 - Add remaining 1 tbsp oil to the skillet. Stir-fry carrots, bell pepper, and mushrooms for 3 to 4 minutes until just tender.
06 - Return beef to the pan. Add noodles, spinach, and sauce. Toss everything together for 2 to 3 minutes until heated through and spinach is wilted.
07 - Serve hot, garnished with toasted sesame seeds and extra sliced scallions.