1. In a small bowl combine the sumac, turmeric, cinnamon, garlic and salt. Place the chicken thighs in a roasting pan and pour the spice mixture over the chicken, using your hands to work the spices into the chicken. Cover and marinate in the fridge at least an hour, up to overnight.
2. When you’re ready to cook, preheat the oven to 280 degrees Fahrenheit.
3. Heat a stainless steel or cast iron pan over high heat and add in the oil. When the oil begins to smoke drop in the chicken, piece by piece, skin side down, taking care not to overcrowd the pan. Cook until the skin crisps, about four minutes, then flip and cook the underside another two minutes.
4. Transfer the cooked chicken back to the pan and repeat with the remaining pieces.
5. While the chicken is cooking, slice the onion and add it into the roasting pan along with the chicken broth.
6. Cover the roasting pan tightly with foil and cook in the preheated oven until the chicken is fork tender, about two to three hours.