In a large mixing bowl, combine Greek yogurt, lemon juice, garlic, paprika, ground cumin, oregano, red pepper flakes, ground cinnamon, and salt. Stir until well blended.

Coat each chicken thigh generously with the marinade and refrigerate for at least 30 minutes, preferably overnight.

Slice the large onion into thick rounds and stack the marinated chicken thighs on top, forming a tower.

Preheat your oven to 375°F (190°C). Bake the assembled chicken and onion for about 1 hour, until the chicken reaches an internal temperature of 165°F.

While baking, slice the red onion and mix with lemon juice, ground sumac, and salt for the topping.

Once the chicken is baked, allow it to rest before slicing and serving atop the onion base with the zesty red onion mixture.