Cook the glass noodles according to package directions. Drain and rinse with cold water. Cut into shorter lengths with scissors and toss with 1 tsp sesame oil to prevent sticking.
In a bowl, marinate the sliced beef with 2 tbsp soy sauce, 1 tsp sugar, and 2 tsp sesame oil for 10 minutes.
In a large skillet or wok, heat 1 tbsp oil. Stir-fry carrots, onions, red pepper, and mushrooms for 2–3 minutes until tender-crisp. Remove and set aside.
Add marinated beef to the skillet and cook until browned. Remove and set aside.
Quickly stir-fry spinach until wilted, then remove.
In a large bowl, combine cooked noodles, vegetables, and beef. Add 3 tbsp soy sauce, 2 tsp sugar, 2 tsp sesame oil, and garlic. Mix well.
Taste and adjust seasoning. Top with sesame seeds before serving.