In a large bowl, combine the olive oil, lemon juice, garlic, and all the spices (cumin, paprika, turmeric, cinnamon, salt, pepper). Optionally, stir in yogurt if you want a creamier marinated chicken. Set aside.
Add the chicken thighs to the marinade and toss until thoroughly coated. For best flavor, you can marinate for 4 hours or overnight in the refrigerator — but you can also proceed right away.
Lightly grease the slow cooker. If using, place a thin layer of sliced onion at the bottom (optional). Then place the marinated chicken (with marinade) into the slow cooker.
Cover and cook on LOW for about 5–6 hours (or HIGH for 3–4 hours) until chicken is tender and easily shreds.
Once cooked, remove the chicken and shred it with two forks. Return shredded chicken to the cooking juices in the pot to soak up extra flavor and moisture.
Serve warm inside pita bread or flatbread with fresh vegetables and your choice of sauce (tahini, yogurt-garlic, hummus, etc.). Optionally add pickles, herbs or other toppings.