In a mixing bowl, blend together Greek yogurt, lemon juice, garlic, paprika, ground cumin, oregano, red pepper flakes, ground cinnamon, and salt to make the marinade.

Coat the chicken thighs thoroughly in the marinade and refrigerate for a minimum of 30 minutes, preferably up to 2 hours.

Slice the large onion into thick rings to create a base. Stack the marinated chicken thighs vertically in a tower formation within the onion rings.

Preheat oven to 400°F (200°C) and bake the chicken shawarma tower for 1 hour or until cooked through.

Combine thinly sliced red onion, lemon juice, ground sumac, and salt for the topping. Let it sit for 10 minutes.

Once baked, cool for a few minutes, slice, and serve garnished with the onion topping.