Cook noodles according to package directions, omitting salt; keep warm.
Heat oil in a large nonstick skillet over medium-high heat. Add garlic; stir-fry 30 seconds. Add chicken; stir-fry 5 minutes. Stir in mushrooms and fish sauce; stir-fry 1 minute. Stir in lo mein noodles, onions, water, and pepper; cover and cook until thoroughly heated (about 2 minutes). Sprinkle with sesame oil.