Heat oven to 350°F. Spray 10-inch nonstick skillet with cooking spray. Add beef and onion; cook in skillet over medium heat 7 to 8 minutes, stirring occasionally, until beef is brown; drain.
In ungreased 3-quart casserole, mix beef mixture and remaining ingredients except frozen vegetables, breaking up tomatoes.
Cover; bake 30 minutes. Stir in frozen vegetables. Cover; bake 30 to 40 minutes longer or until barley is tender.