Squeeze lemon juice into a large plastic zip-lock bag and add lemon pieces. Add thyme, garlic, salt, buttermilk and chicken. Seal bag, removing as much air as possible. Squeeze the bag to fully coat the chicken with liquid.
Refrigerate bag overnight in a large bowl (in case of leaks). Gently manipulate the chicken in the bag at least once during this time to redistribute the marinade.
Preheat oven to 425 degrees. Line a roasting pan or sheet pan with aluminum foil and place a wire rack on top.
Remove chicken from brine and drain thoroughly. Place chicken on the rack, breast side up, tucking in the wings. Sprinkle with salt and pepper, rubbing it into the crevices and under the skin.
Roast until the skin starts to brown, about 25 minutes. Reduce heat to 350 degrees and roast until the internal temperature just inside the thighbone is 165 degrees, about 30-35 minutes. If the chicken gets too brown before reaching the proper temperature, cover with a piece of foil.
Remove from oven and let chicken rest 10 minutes before carving.