Bring the 6 cups of water to a boil in a large pot; add noodles, stir so they do not stick, then cook until tender (see package directions).
Drain noodles and set aside.
In a small bowl, combine 1/2 cup water with cornstarch, fish sauce, soy sauce, and sugar; mix well and set aside.
Heat your wok or large skillet over high heat, then add oil, garlic, and chicken; stir fry until chicken is cooked through; season with salt and pepper.
Stir in carrots and onion; add noodles and stir until noodles are hot.
Stir in bean sprouts; add cornstarch mixture and stir well to thicken sauce.