Before you begin: See the notes below if you plan to dry brine the turkey the day before. Remove the turkey from the refrigerator 1-2 hours before roasting. Remove the neck and giblets, then pat the turkey very dry with paper towels, including inside the cavity.
Preheat oven to 350°F and adjust the oven rack to the lower third position. Place a roasting rack inside the roasting pan. See notes if you don't have a roasting rack.
Place the turkey on the roasting rack then fill the cavity with the onion, celery, garlic, lemon and extra sprigs of herbs and set aside.
18-20 lb turkey, 1/2 medium yellow onion, 2 stalks celery, 6 cloves whole garlic, 1/2 lemon
Add the softened butter, herbs, salt and garlic to a medium microwave safe bowl and mash well with a fork to combine.
1 1/2 cups (3 sticks) unsalted butter, room temperature, 2 tablespoons chopped fresh sage, 2 tablespoons chopped fresh thyme, 1 tablespoon fresh chopped rosemary, 6 cloves garlic, 3 teaspoons salt
Use your fingers to gently loosen the skin over the breasts and thighs, being careful not to tear it. Spread 1/4 of the compound butter underneath the skin and another 1/4 over the outside of the turkey.
Melt the remaining butter in the microwave in 30-second increments. Cut a large piece of cheesecloth approximately 3 1/2 feet by 1 1/2 feet. Submerge it completely in the melted butter, turning and pressing it into the butter until every inch is saturated.
Fold the large piece of cheesecloth in half widthwise to create a double layer of cheesecloth. Drape it over the prepared bird to cover it fully.
Roast for 3 1/2-4 1/2 hours, adjusting the roasting time based on the size of your turkey (see notes). Leave the cheesecloth on for the entire roasting time. It will become very dark as the turkey roasts. This is normal and exactly what you want. There's no need to baste the turkey. If your oven browns unevenly, rotate the pan about halfway through cooking. If you're already opening the oven to rotate it, you can spoon some pan drippings over the cheesecloth if desired.
Begin checking for doneness about 30 minutes before the earliest estimated cooking time. Insert an instant-read thermometer into the thickest part of the breast and the innermost part of the thigh without touching bone. Pull the turkey from the oven when the breast reaches 155-160°F and the thigh reaches 175°F. The breast temperature will continue to rise to 165°F as the turkey rests.
Remove the turkey from the oven and carefully peel away and discard the cheesecloth. If it sticks anywhere, spoon a little warm pan drippings or melted butter over that spot and gently loosen it. Transfer the turkey to a large cutting board or serving platter, loosely tent with foil, and rest for 30 minutes before carving. This is the perfect time to make gravy with the pan drippings.