Boil noodles for 6–7 minutes until soft. Rinse under cold water, drain well, and cut them in half if too long.
Quickly boil spinach for 30 seconds, rinse in cold water, squeeze out excess water, and cut into bite-sized pieces.
Individually stir-fry onion, carrot, bell pepper, and mushrooms in a little oil with a pinch of salt—just until tender. Set each aside.
Fry a thin egg sheet, let it cool, then slice into fine strips for topping.
In a bowl, combine soy sauce, sugar, sesame oil, and a pinch of pepper.
In a large pan or wok, toss noodles, sautéed veggies, spinach, and sauce together on medium heat for 2–3 minutes until warm and coated.
Top with sesame seeds and egg strips (if using). Serve warm or at room temperature.