Steam cabbage head 8 minutes; cool slightly. Remove 16 leaves from cabbage head; discard remaining cabbage. Cut off raised portion of the center vein of each cabbage leaf (do not cut out vein); set trimmed cabbage leaves aside.
Heat oil in a large nonstick skillet over medium heat. Add onion; cover and cook 6 minutes or until tender. Remove from heat; stir in barley and next 4 ingredients (through parsley). Stir in 1/4 teaspoon salt and 1/8 teaspoon pepper.
Place cabbage leaves on a flat surface; spoon about 1/3 cup barley mixture into center of each cabbage leaf. Fold in edges of leaves over barley mixture; roll up. Arrange cabbage rolls in bottom of a 5-quart electric slow cooker.
Combine the remaining 1/4 teaspoon salt, remaining 1/8 teaspoon pepper, apple juice, vinegar, and tomatoes; pour evenly over cabbage rolls. Cover and cook on high 2 hours or until thoroughly heated.