In a large bowl, whisk together plain Greek yogurt, fresh lemon juice, minced garlic, paprika, cumin, turmeric, cinnamon, salt, black pepper, and red pepper flakes. Add chicken thighs, ensuring each piece is thoroughly coated. Seal in a ziplock bag and refrigerate for at least 4 hours, ideally overnight.

Slice one onion into thin rings and arrange them evenly at the bottom of the slow cooker.

Remove the marinated chicken thighs from the ziplock bag, allowing excess marinade to drip off, and place them atop the onions in the slow cooker. Cover and set to high for 3-4 hours or low for 4-6 hours.

Once cooking time is complete, remove the chicken and shred into bite-sized pieces, mixing well with remaining juices in the slow cooker.

To prepare the yogurt sauce, combine plain Greek yogurt, minced garlic, grated cucumber, additional cumin, lemon juice, salt, and pepper in a bowl. Stir until well blended.

Warm pita bread, layer shredded chicken, drizzle yogurt sauce, and garnish with fresh toppings.