Preheat the oven to 325 degrees F (165 degrees C).

Mix together kosher salt, black pepper, and poultry seasoning in a small bowl. Tuck turkey wings under bird and season the cavity with about 1 tablespoon poultry seasoning mixture. Reserve remaining poultry seasoning mixture.

Toss together onions, celery, and carrots in a large bowl. Stuff about 1/2 cup vegetable mixture, sage, and rosemary sprigs into the turkey cavity. Tie legs together with kitchen string. Use your fingers or a small spatula to loosen skin over turkey breast. Place about 2 tablespoons butter under skin and spread evenly. Spread remaining 6 tablespoons butter all over the outside of skin. Sprinkle turkey with remaining poultry seasoning mix.

Spread remaining vegetable mixture into a large roasting pan. Place turkey on top of vegetables. Fill the pan with about 1/2 inch of water. Shape a sheet of aluminum foil over breast of turkey.

Roast turkey in the preheated oven until no longer pink near the bone and the juices run clear, about 3 ½ hours. An instant-read thermometer inserted into the thickest part of the thigh, near the bone, should read 165 degrees F (75 degrees C). Remove the foil during the last hour of cooking and baste turkey with pan juices.

While turkey is roasting, make stock: Place neck and giblets in a medium saucepan with water and bay leaf. Simmer over medium heat for 2 hours. Strain stock and discard neck and giblets. There should be at least 4 cups of stock. Set stock aside.

Remove turkey from the oven. Carefully pour pan juices, about 3 cups, into a shallow bowl. Skim off turkey fat from pan juices, reserving about 2 tablespoons fat. Set aside pan juices. Cover turkey with a doubled sheet of aluminum foil and allow to rest in a warm area for 10 to 15 minutes before slicing.

While turkey is resting, make gravy: Heat 2 tablespoons reserved turkey fat and butter in a large skillet over medium heat. Transfer onion from the roasting pan into the skillet. Cook and stir until onion is browned, about 5 minutes. Stir in flour; cook and stir for about 5 minutes more. Whisk in 4 cups stock and reserved pan juices until smooth; skim off any foam. Stir in balsamic vinegar. Simmer, whisking constantly, until gravy is thickened, about 10 minutes. Stir in sage; season with salt and black pepper.