Preheat oven to 350°F. Thinly slice one lemon and set aside; then peel and juice the other lemon. In a small sauce pot, over low heat, add the butter and when melted, add in the lemon peel and 2 sprigs of thyme and cook until fragrant, about 1 to 2 minute Add in the lemon juice and lightly season with salt.
Remove neck and giblets from chicken. Rinse the chicken under cold water then pat dry with paper towel. Using a syringe, inject half the butter into several spots of the chicken then brush the remaining butter over the chicken. Generously season inside and out with salt and pepper. Evenly sprinkle the garlic powder over the top then place the remaining thyme sprigs and lemon slices on top of the chicken in a single layer.
Carefully place the chicken breast side up on a rack in a shallow roasting pan. Place the chicken in the oven and roast uncovered for 20 min per pound or until juices run clear and a kitchen meat thermometer inserted in the inner thigh registers at least 180°F. (About 2 hours).
Remove the chicken from the oven and let stand a few min before carving.