The day before cooking, sprinkle the turkey very generously inside and out with salt and pepper. Cover and refrigerate.
Remove turkey from refrigerator at least 1 hour prior to cooking to allow it to come to room temperature.
Preheat the oven to 350 degrees F.
Rub the skin of the bird with butter. Stuff the carrot, celery, onion and thyme bunch inside the cavity. Arrange the turkey on a roasting rack set into a roasting pan. Add the chicken stock to the pan. Roast until an instant-read thermometer inserted into the thickest part of the thigh registers 160 degrees F, about 4 hours.
Remove the turkey to a cutting board, tent lightly with foil, and let rest for 20 minutes before carving.
Meanwhile, set the roasting pan over medium-heat high, add the flour and bring to a boil. Cook for 10 minutes, then stir in the sage and 3 tablespoons thyme. Taste and adjust the seasoning. Transfer to a gravy boat to serve with the turkey.