Defrost the turkey. Place the wrapped turkey on a baking sheet and set on the bottom shelf of the fridge. Defrost for 1 day if fresh, or 4-5 days, if frozen. For a quicker defrost, submerge the turkey in a large sink or cooler filled with cold water and defrost for 30 minutes per pound of turkey.
1 day before cooking, brine the turkey. Measure the salt into a small bowl. Set up your work station with kitchen shears, a plastic bag, a fresh baking sheet with a wire rack set over it, and a garbage can nearby.
Cut the packaging off the defrosted turkey, being careful not to cut the skin or flesh, and remove from the bag. Set the turkey on the baking sheet with the wire rack. Cut off any excess skin around the neck and release the plastic holder securing the legs. Remove any innards and giblets from the cavity and discard. Thoroughly pat the turkey dry with paper towels.
Sprinkle the salt all over the turkey, making sure to get in the cavity, under the skin, and between any crevices.
Refrigerate the turkey, uncovered, for 1 day. If brining for more than 1 day, cover the turkey with plastic wrap, then remove for the last day to let the skin dry out before roasting.
A few hours before serving, prepare the turkey for roasting. Set an oven rack at the lowest height and the other in the middle of the oven. Preheat the oven to 325˚F (160˚C).
Add the butter to a medium bowl. Chop the sage leaves and add to the bowl, along with the garlic, thyme, and rosemary. Mash the herbs into the butter. Set aside.
Set a roasting pan or single-use aluminum pan over a baking sheet near your work station.
Trim off the wing tips of the turkey and add to the roasting pan. Cut off any excess skin to expose the wishbone, then use a paring knife to cut out the bone. Use your hands or kitchen shears if the bone is hard to remove. Add to the roasting pan.
Turn the bird so the legs are facing you. Tug out the plastic holder. Loosen the skin around the breasts, using your fingers to create a pocket. Be careful not to tear the skin. Rub the herb butter under the skin, then all over the rest of the turkey. Make sure to get in any crevices and the cavity.
Stuff the lemon wedges and ¼ of the onion inside the turkey cavity. Add the rest of the onion wedges to the roasting pan, along with the carrot and celery. Pour the chicken broth into the pan.
Use the wire rack to rest the turkey over the roasting pan, then pour any accumulated juices from the baking sheet into the pan.
Set the roasting pan on the lowest oven rack. Place the turkey on the wire rack on the middle oven rack, directly over the roasting pan to catch any juices.
Roast the turkey for about 2 hours, or until the internal temperature reaches 160˚F (71˚C). Start checking the turkey about 90 minutes into roasting to ensure it does not overcook.
Remove the turkey from the oven and set the wire rack over a baking sheet to catch any juices. Let rest for about 30 minutes while the internal temperature climbs to 165˚F (74˚C).
Meanwhile, make the gravy: Remove any solids from the roasting pan. Strain the liquid into a large pot and bring to a boil, then let reduce to your desired consistency. The gravy can be served as is or, for a thicker gravy, ladle a few spoonfuls of stock into a small bowl with the flour. Whisk until no lumps remain, then stir a bit of the flour mixture at a time back into the pot. Return to a boil and let cook for 5 minutes, until thickened. Repeat as needed until the gravy has reached your desired consistency. Reduce the heat to low, whisking occasionally, until ready to serve, or remove from the heat and bring back to temperature before serving.
Carve the turkey: Transfer the turkey to a large cutting board with a lip to catch any juices. First, release the legs to make carving the rest of the bird easier. Using a sharp knife, cut along where the legs meets the breasts, being careful not to cut all the way through. Then, use your hands to pull the legs and thigh away from the carcass and tuck them under the wings to keep them in place.
Cut straight down along both sides of the breast bone until you can’t cut any further, then cut horizontally near the wing to release the breasts from the carcass.
Cut all the way through the legs to release from the hip joint.
Find where the shoulder meets the breast, then cut through that joint to remove the wings, using your hands to pull the wing away if the joint is tough to find.
Set the carcass aside--you can pick off any remaining meat for leftovers, or make stock from the whole carcass.
Slice the breasts crosswise, against the grain, then transfer to a serving platter. Cut the drumsticks from the thighs through the joint, wiggling the knife to find an easy release or pulling apart with your hands. Add the drumsticks to the platter. Cut the thigh meat away from the bone, then into bite-size pieces, and add to the platter. Release the flats from the wings by cutting through the joint, pulling apart with your hands if necessary, then add to the platter.
Rearrange the meat on the platter so the larger pieces of meat with crispy, browned skin are on top. Garnish the platter with sage leaves and lemon wedges, if desired.