Place the roaster oven on a large wooden cutting board or other heat-safe surface. It's going to get hot, so leave plenty of room around it. Remove the rack and the inner liner and put the lid back on. Preheat the roaster oven to the hottest temperature, usually 450° to 500° F.
While the oven heats, prepare the turkey. Remove the neck and giblets and use the neck to make turkey broth for the gravy. Loosen the skin around the turkey breast and tuck one large sage leaf under the skin on each side. Pat the skin back down. Rub the turkey all over with the olive oil. Tuck the lemon half into the rib cavity and stuff in the thyme, parsley, and rosemary. Place the turkey on the rack in the roaster oven liner. Sprinkle the kosher salt and black pepper all over the bird. Insert the thermometer probe deep into the thigh muscle, but not touching the bone. Leave the probe cable hanging outside the liner.
Place the liner and turkey in the preheated roaster oven and cover. Connect the probe cable to the digital thermometer and set the temperature alarm for 165° F, the minimum safe internal temperature recommended by the USDA. You can set it higher according to your personal preference. Cook for 30 minutes, then reduce the temperature to 325° F.
When the internal temperature reaches 165° F or your preferred temperature, turn off the oven and lift the bird onto a cutting board or platter. Leave the temperature probe in and cover the bird with foil. Allow to rest for 15 to 20 minutes to allow the juices to soak back into the meat, then remove the herbs and lemon, and carve.