Place the onions and garlic in a microwave safe bowl. Microwave, stirring occasionally, until the onions are softened, 3 to 5 minutes. Transfer them to the slow cooker.
Coat the chicken thighs with the lemon pepper, oregano, allspice and salt and add to the slow cooker.
Cover and cook on LOW for 6 to 8 hours, or until the chicken is tender.
Remove chicken from slow cooker and place on a cutting board. Stir the yogurt into the onion mixture in the slow cooker. Shred the chicken with 2 forks. Season to taste with salt and pepper. Stir back into the slow cooker.
To serve, with a slotted spoon, spoon the chicken mixture into the warm pitas. Top with cucumber, tomato and feta cheese if using.