Cook the macaroni: Bring a pot of salted water to boil. Add macaroni and cook until just al dente. Drain and set aside.
Prepare the beef: In a bowl, combine beef slices with 1 tablespoon soy sauce, black pepper, and half the minced garlic. Let marinate for 10–15 minutes.
Stir-fry the vegetables: Heat 1 tablespoon oil in a skillet over medium-high heat. Add onions, carrots, bell peppers, and stir-fry for 2–3 minutes until slightly soft. Remove and set aside.
Sear the beef: In the same skillet, add a bit more oil if needed. Add marinated beef and stir-fry until browned, about 2–3 minutes.
Combine everything: Add macaroni and vegetables back into the skillet with beef. Stir in the remaining soy sauce, oyster sauce, sugar, and rest of garlic. Mix well and stir-fry for another 2–3 minutes.
Finish and garnish: Taste and adjust seasoning. Remove from heat, garnish with chopped green onions, and serve hot.