Preheat the oven to 400 F (200 C).
In a small bowl, add the shallots, garlic and black pepper to taste. Stir until evenly mixed and set aside.
Arrange carrots, celery and onion in the bottom of a roasting pan.
Unwrap the turkey from its package and remove the neck, giblets and fatty tissues from body and neck cavities. Discard giblets and fatty tissues; reserve the neck for the stock. Rinse the turkey inside and out. Pat dry with paper towels.
Place the turkey breast-side up on top of the vegetables in the roasting pan. Tuck the wing tips behind the back of the turkey. Rub the turkey with the shallot mixture. Arrange tomatoes around the outside of the turkey. Place into the middle of the oven uncovered.
Bake for 20 minutes at 400 F (200 C), then reduce the oven temperature to 325 F (170 C). Check the doneness after the bird has roasted about 3 to 3 1/2 hours. Using a food thermometer, check the temperature in the thigh, under the wing and in the thickest part of the breast. The minimum internal temperature should be 165 F (74 C).
While the turkey is in the oven, put the turkey neck into a small saucepan. Add 4 cups of water. Place over medium heat, cover and simmer while the turkey roasts, for at least an hour.
For the gravy
Remove the turkey from the roasting pan and set onto a platter. With a slotted spoon, remove the vegetables and arrange around the turkey. Cover the turkey and vegetables tightly with aluminum foil and set aside until you're ready to eat.
Place the roasting pan onto the stove top over medium heat. Add 2 cups of water into the pan and stir about 5 minutes or until the drippings and browned bits from bottom dissolve.
Place a strainer over a fat separator cup or bowl. Pour pan drippings through strainer. Add enough liquid stock to make 4 cups total.
To remove fat from drippings, add several ice cubes to the liquid and place stock into freezer for 10 minutes. If you're using a fat separator cup, pour off stock into a saucepan. If you're using a bowl, remove hardened fat with spoon and place stock into a saucepan. Expect to have about 4 cups of liquid. Place the saucepan onto the stove top over medium heat.
Place saucepan over medium heat and bring to a simmer. Add thyme and sage to the simmering stock. Continue to simmer until the stock is reduced by 1/4, or until about 3 cups of stock remain.
Pour milk into a small bowl. Add cornstarch and stir to mix evenly. Slowly pour milk mixture into the simmering stock, stirring slowly. Bring sauce to a boil and continue to stir until stock thickens and has a nice shine, about 3 to 5 minutes.
Pour gravy into warmed gravy boat and serve with turkey and roasted vegetables.