Preheat the oven to 325°F (165°C).
Combine the kosher salt, black pepper, and poultry seasoning in a small bowl. Tuck the turkey wings underneath the bird. Season the cavity with about 1 tablespoon of the seasoning mixture and reserve the remainder.
Combine the chopped onions, celery, and carrots. Place about ½ cup of the vegetables inside the turkey cavity along with some fresh sage and the rosemary sprigs.
Tie the turkey legs together using kitchen twine. Carefully loosen the skin over the breast with your fingers or a small spatula. Spread about 2 tablespoons of softened butter underneath the skin.
Rub the remaining 6 tablespoons of butter over the outside of the turkey. Sprinkle evenly with the remaining seasoning mixture.
Arrange the remaining chopped vegetables across the bottom of a large roasting pan. Set the turkey on top and add enough water to reach approximately ½ inch up the bottom of the pan. Loosely cover the turkey breast with aluminum foil.
Roast for approximately 3½ hours, or until the thickest part of the thigh reaches a safe internal temperature of 165°F (74°C). Remove the foil during the final hour and occasionally baste the turkey with the pan juices.
Meanwhile, prepare the stock. Place the reserved turkey neck and giblets in a saucepan with 6 cups of water and the bay leaf. Simmer gently for about 2 hours. Strain and reserve at least 4 cups of stock.
Remove the turkey from the oven. Carefully transfer approximately 3 cups of pan juices to a heat-safe bowl. Skim the fat from the juices, reserving about 2 tablespoons.
Loosely cover the turkey with foil and allow it to rest for at least 15 minutes before carving. A slightly longer rest of 20–30 minutes is also suitable for a large bird.
To finish this Roast Turkey and Gravy, heat the reserved turkey fat with 1 tablespoon butter in a large skillet over medium heat. Add some of the roasted onion from the pan and cook until nicely browned.
Stir in the flour and cook for about 3–5 minutes, stirring frequently. Gradually whisk in 4 cups of the prepared turkey stock followed by the reserved pan juices.
Bring the gravy to a gentle simmer and cook, whisking regularly, until smooth and thickened, about 10 minutes. Stir in the optional balsamic vinegar and chopped fresh sage.
Taste and season the homemade turkey gravy with salt and black pepper as needed. Carve the rested turkey and serve with the hot gravy.