Remove and reserve the neck and giblets from the turkey cavity. Place the turkey, breast side down, on a sturdy work surface. Use a sharp knife or poultry/kitchen shears to cut through the bones along the sides of the backbone. (Reserve the backbone for the gravy.)
Flip the turkey over and press the breast firmly with the heel of your hands until the breastbone breaks, flattening the turkey on the surface.
Place a wire rack in a lightly greased roasting pan or rimmed baking sheet large enough to hold the turkey. Arrange the celery and onion pieces around the pan.
Mix together the softened butter, herbs, and garlic in a small bowl. Pat the turkey dry with paper towels, both on the skin and in between the skin and the flesh. Rub the butter mixture underneath the skin of the breast, legs, and thighs.
Place the turkey in the roasting pan on the wire rack. Tuck the wing tips under and stretch the legs of the turkey to allow the bird to lay flat. Use a meat injector to inject the turkey marinade into the meaty parts of the breast, wings, thighs, and legs. Sprinkle with salt and pepper.
Insert a meat thermometer in the meaty part of the thigh and roast at 425°F until the turkey is golden brown and registers 165°F, about 1 hour and 20 minutes, tenting with aluminum foil to prevent excess browning if necessary. Transfer the turkey to a rimmed baking sheet to rest while making the gravy.
While the turkey roasts, place the turkey neck, giblets, and backbone in a large saucepan and cover with water. Bring to a boil; reduce the heat to medium-low and simmer for 35 to 40 minutes or until the meat and giblets are tender.
Strain the broth through a wire mesh strainer into liquid measuring cups and reserve. Let the neck, giblets, and backbone cool. Remove the meat from the bones and the chop along with the giblets.
Finish the gravy: Strain the pan drippings through a mesh strainer into a large liquid measuring cup. Skim off excess fat from the top. Place the roasting pan over two burners. Add ¼ cup butter to the pan and melt over medium heat.
Whisk in the flour and cook until foamy. Pour in the reserved pan drippings and whisk until smooth. Gradually add 2 to 3 cups of reserved broth, whisking until the gravy reaches the desired consistency. (Add boxed broth if necessary.) Stir in the reserved chopped meat and giblets, if desired. Serve with the turkey.