First, spatchcock your chicken by cutting the spine out and pressing firmly with your palm on the breast bone to break the breast bone. Spatchcocking the chicken enables you to roast your bird more quickly.
Once spatchcocked, sprinkle the kosher salt over every inch of skin and place on top of a cookie sheet in the fridge and let it chill for 8 hours until you're ready to cook.
When ready to cook, remove chicken from fridge and rinse and dry thoroughly before seasoning.
Mix the sumac, thyme, pepper, and garlic salt in a small bowl.
Using your hands, rub the olive oil over every inch of the skin. Next sprinkle the seasoning on the skin and press firmly in with your hands. You should be able to cover all the skin with your mix.
When ready to cook, place your cast iron in the oven and bring to 450°F. Once the oven has reached temperature, carefully remove the cast iron from the oven and add olive oil to lubricate the bottom.
Carefully put the chicken skin side down, the pan should sizzle.
Put the cast iron back in the oven and cook for 45 minutes and add two halves of the lemon.
After 45 minutes, remove the cast iron and carefully flip the chicken with tongs and a spatula. Cook for an additional 15 minutes, skin side up or until a thermometer inserted in the breast reads 160°F.
Remove from heat and squeeze roasted lemon on the chicken when ready to serve.