How to Make It
Grate zest from lemon to equal 1 teaspoon. Cut lemon in half, and set aside. Combine buttermilk, oil, 4 smashed garlic cloves, chopped thyme, 1 tablespoon salt, pepper, and lemon zest in a large ziplock plastic freezer bag. Seal bag, and shake to evenly distribute. Add chicken to bag; seal and place chicken, breast side down, in refrigerator. Chill 8 hours or up to overnight, shaking bag occasionally.
Preheat oven to 300°F with rack in middle of oven. Remove chicken from marinade, and pat dry with paper towels. Discard marinade. Season chicken with remaining 1 tablespoon salt, using 1 teaspoon in cavity and 2 teaspoons on outside of chicken. Stuff cavity with thyme sprigs, lemon halves, and remaining 2 smashed garlic cloves. Tie legs together with kitchen twine, and tuck wing tips under; transfer to a large cast-iron skillet. Place a sheet of parchment paper over chicken. Top with 2 large sheets of aluminum foil, and crimp around edges of skillet to seal.
Bake in preheated oven until a meat thermometer inserted into thickest part of thigh registers 150°F, 1 hour and 20 minutes to 1 hour and 30 minutes. Remove chicken from oven, and discard parchment paper and foil. Increase oven temperature to broil; let oven preheat 5 minutes.
Baste chicken with pan juices. Return chicken to oven, and broil until skin is dark golden brown, 10 to 12 minutes, basting once halfway through. Let rest 10 minutes. (Internal temperature will continue to rise to 165°F.) Carve chicken, and drizzle pan drippings over sliced meat. Garnish with thyme leaves.