Preheat the oven to 325°F. Line a roasting pan with a rack. If you don't have a rack, you can line the pan with carrots or celery sticks to keep the turkey off the bottom of the pan.
Check the cavity of the turkey and remove the neck and giblets if present. Reserve the neck, heart, and gizzard. Discard the liver. Pat the turkey dry with paper towels and place in the roasting pan. Stuff the turkey cavity with the halved onion, lemon, and a small bunch of sage and thyme leaves. Tie the legs together and tuck the wing tips under the turkey.
In a small bowl, combine the softened butter, chopped sage and thyme, kosher salt, and pepper. Mix together. Spread the herbed butter evenly over the turkey. Add 1 cup of water to the roasting pan and cover either with a lid or foil. If using foil, place a piece of parchment paper directly on the turkey and then cover tightly with the foil.
Roast the turkey, covered, for about 2 hours. Remove the foil and baste the turkey. Continue roasting the turkey, basting every half hour, until the turkey is fully cooked, another 1 to 2 hours based on size. The turkey should register an internal temperature over 165°F in the thickest part of the breast and thigh. The juices inside the cavity should run clear; if they are still pink, continue roasting until they run clear.
In a medium saucepan, add the turkey neck, heart, and gizzard along with the chopped onion, carrot, and celery. Tie the thyme, parsley, and bay leaves together to make a bouquet garni and add to the saucepan along with the kosher salt and peppercorns. Add 8 cups of water and bring to a boil. Reduce the heat and simmer on low for about an hour. Skim the scum and foam from the surface as it cooks. Strain and reserve the broth.
When the turkey is done, remove to a platter or cutting board and tent with foil. Strain the juices from the roasting pan. Skim off and reserve ¾ cup of the fat and return it to the roasting pan. Skim and discard the remaining fat from the pan juices. Add enough of the reserved giblet broth to the pan juices to make 6 cups.
Place the roasting pan over medium heat. Add the flour to the roasting pan and whisk into the fat to make a roux. Let the roux cook for 2 to 3 minutes. Add the reserved pan juices and giblet broth, a cup at a time, whisking constantly to prevent lumps. Bring the gravy to a boil and cook until thickened. Taste the gravy and adjust the seasoning if needed. Strain the gravy and reserve until serving.
Allow the turkey to rest for 20 to 30 minutes before carving. To carve the turkey, use a sharp knife to separate the leg from the body. Grasp the thigh and bend it backwards to expose the joint and then use the knife to remove the whole leg. Repeat on the other side. Pull the leg meat from the bones and transfer to a serving platter.
To remove the breast, cut down along the breastbone, following the curve of the rib bones. Gently pull the breast off the bones as you cut. Repeat on the other side. Slice the turkey breasts and transfer to the serving platter. Pull the wings off, using the knife to separate them at the joint. The turkey carcass can be set aside to make turkey broth if desired.
Serve the turkey with the gravy and enjoy!