In a large mixing bowl, whisk together Greek yogurt, lemon juice, minced garlic, and spices. Place chicken thighs in a ziplock bag with marinade, seal, and refrigerate for at least 4 hours, preferably overnight.

Slice the onion and spread it evenly at the bottom of the slow cooker. Layer marinated chicken on top without crowding.

Cover and cook on high for 3-4 hours or low for 4-6 hours until chicken shreds easily.

Carefully remove chicken and shred into bite-sized pieces. Return to slow cooker to soak up juices for 10 minutes.

Prepare yogurt sauce by mixing additional Greek yogurt, minced garlic, grated cucumber, cumin, lemon juice, salt, pepper, and red pepper flakes. Chill before serving.

Warm pitas and fill with shredded chicken. Drizzle yogurt sauce on top and add fresh toppings.