01 - Toss the sliced beef with 1 tbsp soy sauce, cornstarch, and 1 tsp sesame oil in a bowl. Let marinate while preparing other ingredients.
02 - Cook noodles according to package instructions. Drain, rinse under cold water, and set aside.
03 - Whisk together all sauce ingredients 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 2–3 minutes until just cooked. Transfer to a plate.
05 - Add remaining 1 tbsp oil to the skillet. Stir-fry carrots, bell pepper, and mushrooms for 3–4 minutes until just tender.
06 - Return the beef to the pan. Add noodles, spinach, and sauce. Toss everything together for 2–3 minutes until heated through and spinach is wilted.
07 - Serve hot, garnished with sesame seeds and extra scallions.