We stuffed the bird with mushroom stuffing, but you can use your own favorite recipe if you prefer.
Preheat oven to 350 degrees, with rack in lowest position. Prepare and stuff the turkey.
Cut turkey neck into pieces; combine in a large roasting pan with carrots, onions, and reserved sage stems. Set rack in pan (over vegetables); place turkey on rack, breast side up. Rub turkey with butter; season generously with salt and pepper.
Place pan in oven. Pour 6 cups water in bottom of pan; tent with foil. Roast, basting bird every 30 minutes with pan liquids, until an instant-read thermometer inserted in thickest part of thigh (avoiding bone) registers 125 degrees, about 3 1/2 hours.
Remove foil; raise oven heat to 400 degrees. Continue roasting, basting occasionally, until thigh reaches 170 degrees, 45 to 60 minutes more. Tent with foil if bird browns too quickly; add more water if pan becomes dry. Transfer turkey (on rack) to a rimmed baking sheet; cover loosely with foil while making the gravy.