In a small bowl, whisk together olive oil, minced garlic, cumin, coriander, smoked paprika, turmeric, cinnamon, cayenne if using, salt, pepper, and lemon juice until a smooth marinade forms.

Spread the sliced onion in an even layer across the bottom of the slow cooker.

Place the chicken thighs on top of the onions. Pour the marinade over the chicken and turn each piece to coat thoroughly. For best flavor, cover and marinate in the refrigerator overnight before cooking.

Cover the slow cooker and cook on LOW for 6 hours or HIGH for 3 hours, until the chicken is tender and reaches an internal temperature of 165 degrees F. Do not lift the lid during cooking.

Using two forks, shred the chicken directly in the slow cooker and stir it back through the cooking juices. Optional: spread shredded chicken on a baking sheet and broil for 3 to 4 minutes for crispy edges.

Serve in warm pita bread or over steamed rice, garnished with fresh parsley and drizzled with yogurt sauce or tahini.