Spray 3- to 4-quart slow cooker with cooking spray. In cooker, mix onion, garlic, lemon-pepper seasoning, oregano and allspice. Add chicken; coat with seasoning mixture.
Cover; cook on Low heat setting 6 to 8 hours.
About 10 minutes before serving, heat pita bread folds as directed on package. Meanwhile, remove chicken from cooker; place on plate. Stir yogurt into onion mixture. Shred chicken by pulling apart with 2 forks. Spoon chicken onto warm pita breads. Spoon onion mixture, using slotted spoon, onto chicken. Top with tomato and cucumber. Fold pita breads in half to serve.