Remove the completely thawed turkey from the refrigerator. Remove the neck and giblets from the cavity and pat the turkey thoroughly dry with paper towels.
Preheat the oven to 325°F. Place a roasting rack inside a shallow roasting pan and lightly grease the rack.
Place the onion, oranges, garlic, and rosemary loosely inside the turkey cavity. Do not pack the cavity tightly.
Place the turkey breast-side up on the prepared roasting rack. Tuck the wing tips underneath the turkey and loosely tie the legs together with kitchen twine.
Season the outside of the turkey with kosher salt and black pepper. If the turkey has been brined, pre-salted, or injected with a salt solution, use less additional salt.
Loosely tent the breast with aluminum foil. Place the turkey in the oven and roast for 1 1/2 hours.
Carefully remove the foil and generously brush the turkey with the melted butter. Return the turkey to the oven, uncovered, and continue roasting until golden brown and cooked through, approximately 1 1/2 to 2 1/4 additional hours. Baste occasionally with the pan juices, if desired.
Begin checking the turkey after approximately 3 hours of total roasting time. The turkey is finished when a meat thermometer registers at least 165°F in the thickest part of the breast, the innermost portion of the thigh, and the innermost portion of the wing. Make sure the thermometer does not touch the bone.
Carefully transfer the turkey to a large cutting board. Loosely tent it with foil and let it rest for at least 20 minutes before carving.
While the turkey rests, pour the pan drippings into a fat separator. Measure 1/4 cup of the separated turkey fat into a medium saucepan. If you do not have enough turkey fat, add enough unsalted butter to equal 1/4 cup.
Place the saucepan over medium heat. Add the flour and whisk constantly for 1 to 2 minutes, until the mixture is smooth and lightly golden.
Measure the defatted turkey drippings and add enough turkey or chicken stock to make 2 cups of total liquid. Slowly pour the liquid into the saucepan while whisking continuously.
Bring the gravy to a gentle simmer and cook, whisking frequently, until it reaches your desired consistency. Season with kosher salt and black pepper to taste.
Carve the rested turkey and serve it immediately with the warm homemade gravy.