Preheat oven to 450˚F.
Place butter, salt, pepper, lemon zest, minced thyme, and garlic into a small bowl and mix together until evenly combined. Set aside.
Turn chicken breast-side down on a clean cutting board. Using sharp kitchen scissors, cut out backbone of chicken.
Turn chicken over and open back, so the legs are facing inward.
Place palms over the center of the breastbone and firmly press down to crack (this will allow the chicken to lay flat).
Using fingers, gently pull skin away from the meat throughout chicken. Then carefully and evenly spread half of butter mixture under skin of chicken.
Rub remaining butter evenly over entire skin of chicken.
Tuck wing tips behind chicken.
Place prepared chicken into a 12” cast iron skillet, turning legs inward.
Transfer skillet to preheated oven and roast for 15 minutes.
Reduce temperature to 400˚F and continue to roast for an additional 50 to 55 minutes or until the internal temperature reads 165˚F when a thermometer is inserted into the thickest part of the leg, and the juices run clear.
Remove skillet from oven and allow chicken to rest, about 10 minutes.
Top chicken with thyme leaves and sliced chives.