Beef Barley Stew

1 pound extra-lean ground beef
1/2 cup chopped onion (about 1 medium)
2 cups beef broth
2/3 cup uncooked barley
1/2 teaspoon dried oregano
1/4 teaspoon salt
1/4 teaspoon pepper
1 can (16 ounces) whole tomatoes, undrained
1 can (8 ounces) sliced water chestnuts, undrained
1 package (10 ounces) frozen mixed vegetables

Heat oven to 350 degrees. Spray 10-inch nonstick skillet with nonstick cooking spray.
Cook ground beef and onion in skillet over medium heat, stirring occasionally, until beef is brown; drain.
Mix beef mixture and remaining ingredients except frozen mixed vegetables in 3-quart casserole; break up tomatoes. Cover and bake 30 minutes.
Stir in mixed vegetables. Cover and bake 30 to 40 minutes longer or until barley is done.