In a medium bowl, whisk together Greek yogurt, lemon juice, minced garlic, paprika, cumin, black pepper, salt, turmeric, cinnamon, and red pepper flakes until fully combined.

Place the chicken thighs in a large ziplock bag and pour the marinade over them, sealing the bag tightly. Refrigerate for at least 4 hours, or ideally overnight.

Slice the onion and layer it at the bottom of the slow cooker, then place the marinated chicken thighs on top.

Cover the slow cooker and cook on high for 3-4 hours or low for 4-6 hours until the chicken is fork-tender.

Once cooked, use tongs to remove the chicken and shred it, placing it back into the slow cooker to warm in the juices for about 10 minutes.

Prepare the yogurt sauce by mixing together Greek yogurt, minced garlic, grated cucumber, cumin, lemon juice, salt, black pepper, and optional red pepper flakes.

Warm the pitas and fill each with shredded chicken and yogurt sauce, adding toppings as desired.