Prepare barley according to package directions. In a small bowl, combine water, soy sauce and cornstarch until smooth; set aside.
In a large skillet or wok, saute garlic in oil for 15 seconds. Add carrots and beans; stir-fry for 2 minutes. Add onions; stir-fry 1 minute longer. Stir soy sauce mixture; stir into skillet. Bring to a boil; cook and stir until thickened, about 1 minute. Add barley; heat through. If desired, stir in cashews.