Coat the inside of a slow cooker with non-stick cooking spray.
Place the chicken in the bottom of the slow cooker. Top with red onions, garlic, lemon juice, Greek yogurt, olive oil, red wine vinegar, and seasonings. Use the back of a spoon or spatula to spread over chicken.
Cover and cook on LOW for 4 hours. If chicken is frozen cook on LOW for 6 hours.
Remove the lid and shred the chicken with two forks. Keep the chicken in the juices it cooked in until ready to serve.
Place chicken on warm pita bread or Naan. Top with lettuce, tomatoes, red onion, cucumbers, olives, Tzatziki sauce and feta cheese.