Cook the lo mein noodles according to package instructions. Drain and rinse under cold water, then set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced chicken breast to the skillet in a single layer. Cook undisturbed for 3-4 minutes until browned, then flip and cook for another 3-4 minutes until no longer pink.
Add minced garlic and stir for about 30 seconds until fragrant.
Add sliced bell peppers and julienned carrots. Stir-fry for 3-4 minutes until tender yet crisp.
Add the cooked lo mein noodles and toss with chicken and vegetables.
Pour soy sauce and sesame oil over the mixture, tossing gently to coat.
Season with black pepper and stir-fry for an additional 2 minutes.
Add chopped green onions and optional bean sprouts. Stir for another minute.
Remove from heat and serve hot, garnished with extra green onions if desired.
Notes
Store leftovers in an airtight container in the refrigerator for up to three days. Reheat in the microwave or on the stovetop.