Marinate the beef: Combine beef, minced garlic, sugar, ground black pepper, soy sauce, and sesame oil in a bowl. Refrigerate while preparing other ingredients.
Prepare the egg garnish: Separate the egg yolk and white. Add a pinch of salt to each and fry in a thin layer. Let cool and slice into thin strips.
Blanch the spinach: Cook spinach in boiling water for 30 seconds to 1 minute, then squeeze out excess water. Season with soy sauce and sesame oil.
Cook the noodles: Boil sweet potato starch noodles until chewy. Drain and cut into manageable lengths.
Stir-fry vegetables and meat separately: Stir-fry onions, green onions, mushrooms, carrot, and red bell pepper individually with a pinch of salt. Then stir-fry the marinated beef and mushrooms. Combine all in a large bowl.
Mix everything: Add the cooked noodles to the bowl with vegetables and beef. Season with sesame oil, soy sauce, and sugar. Mix well.