Preheat your oven to 425°F (220°C). Position an oven rack in the lowest setting. Remove the turkey from its packaging and remove the neck and giblets from the cavity. Pat the turkey completely dry using paper towels. Place the turkey in a large roasting pan fitted with a rack.
In a small bowl, combine the softened butter, chopped sage, thyme, rosemary, 2 teaspoons of salt, and 1 teaspoon of pepper. Mix until well incorporated.
Gently separate the skin from the breast meat using your fingers, starting at the neck cavity. Be careful not to tear the skin. Rub about half of the herb butter mixture underneath the skin, evenly coating the breast meat.
Rub the remaining herb butter mixture all over the exterior of the turkey, including the legs and wings.
Stuff the turkey cavity with the quartered onion, carrots, celery, halved garlic, and halved lemon. Tie the legs together loosely with kitchen twine (trussing).
Pour about 2 cups of water or chicken broth into the bottom of the roasting pan to prevent drippings from burning.
Place the turkey in the preheated oven at 425°F for 30 minutes to achieve a nice browning.
Reduce the oven temperature to 325°F (160°C). Continue roasting for 3 to 3½ hours, or until a meat thermometer inserted into the thickest part of the thigh (without touching bone) reads 165°F (74°C).
If the skin begins to brown too quickly, loosely tent the turkey with aluminum foil.
Once cooked, remove the turkey from the oven. Tent it tightly with foil and let it rest for at least 30 minutes before carving. This allows the juices to redistribute, ensuring a moist turkey.
While the turkey rests, make the pan gravy. Pour the drippings from the roasting pan into a measuring cup and let the fat rise to the top. Skim off ¼ cup of fat and return it to the pan.
Place the pan on the stovetop over medium heat. Whisk in ¼ cup of flour and cook for 2 minutes to make a roux.
Slowly whisk in 2 to 3 cups of turkey or chicken broth, stirring constantly until smooth and thickened. Season with salt and pepper to taste.
Carve the turkey and serve with the warm rosemary-sage pan gravy.