In a large bowl, combine cumin, coriander, paprika, turmeric, cinnamon, cayenne pepper, minced garlic, olive oil, lemon juice, salt, and 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 sliced onion at the bottom of the slow cooker. Add the marinated chicken thighs on top. Pour the chicken broth over the chicken.
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the chicken is tender and easily shreds with a fork.
Once cooked, shred the chicken in the slow cooker using two forks, mixing it with the juices.
Serve the chicken in pita bread or flatbreads, topped with cucumbers, tomatoes, red onion, and a drizzle of tahini sauce.