In a large bowl, combine the olive oil, cumin, coriander, smoked paprika, turmeric, cinnamon, cayenne pepper, garlic, salt, and black pepper. Mix well to create a marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 1 hour, or overnight for best flavor.
Place the marinated chicken in the slow cooker. Pour any remaining marinade over the chicken.
Cook on low for 6-8 hours or on high for 3-4 hours, until the chicken is tender and easily shredded.
Once cooked, shred the chicken using two forks and mix it with the juices in the slow cooker.
Stir in the yogurt and lemon juice until well combined.
Serve the shawarma chicken in pita bread or over rice, garnished with fresh parsley.