In a medium bowl, whisk together plain Greek yogurt, fresh lemon juice, minced garlic, and all the spices: ground cumin, paprika, turmeric, cinnamon, black pepper, salt, and red pepper flakes.

Place the chicken thighs in a large ziplock bag and pour the marinade over them, sealing the bag tightly. Massage the marinade into the chicken.

Slice the onions thinly and layer them on the bottom of your slow cooker. Place the marinated chicken on top of the onions.

Set your slow cooker to high for 3-4 hours or low for 4-6 hours.

Once cooked, shred the chicken directly in the slow cooker with two forks and let it sit for an additional 10 minutes.

Combine the additional Greek yogurt, grated cucumber, minced garlic, cumin, and lemon juice in a small bowl.

Warm pitas or flatbreads, layer the shredded chicken, add yogurt sauce, and garnish with toppings like lettuce and tomatoes.