In batches, cook cabbage in boiling water 3-5 minutes or until crisp-tender. Drain; cool slightly. Trim the thick vein from the bottom of each cabbage leaf, making a V-shaped cut., In a large bowl, combine rice, onion, egg, milk, salt and pepper. Add beef; mix lightly but thoroughly. Place about 1/4 cup beef mixture on each cabbage leaf. Pull together cut edges of leaf to overlap; fold over filling. Fold in sides and roll up., Place 6 rolls in a 4- or 5-qt. slow cooker, seam side down. In a bowl, mix sauce ingredients; pour half of the sauce over cabbage rolls. Top with remaining rolls and sauce. Cook, covered, on low 6-8 hours or until a thermometer inserted in beef reads 160° and cabbage is tender.