For the turkey
1 16- to 18-lb. turkey (preferably fresh)
1/2 medium lemon
1 medium head garlic, separated into cloves and peeled
4 oz. (1/2 cup) unsalted butter, softened
1 Tbs. finely chopped fresh thyme
Freshly ground black pepper
4 sprigs fresh sage (7 to 8 inches long)
4 sprigs fresh rosemary (6 to 7 inches long)
2 medium carrots, chopped
2 large celery ribs, chopped
4 medium shallots, chopped
1 dried bay leaf
For the gravy
5 Tbs. all-purpose flour
1 cup lower-salt chicken or turkey broth; more as needed
1 tsp. finely chopped fresh thyme leaves
1/2 tsp. finely chopped fresh rosemary leaves
1/4 tsp. finely chopped fresh sage leaves
Freshly ground black pepper
1 Tbs. unsalted butter
Roast the turkey
1. Position a rack in the bottom of the oven and heat the oven to 325°F. Rinse and dry the turkey. Rub the inside of the body and neck cavities with the cut side of the lemon half and sprinkle 1 tsp. salt inside the cavities.
2. Thinly slice three garlic cloves. Carefully slide your hands under the skin of the turkey to loosen it from the breast. Push the garlic slices between the skin and the breast, being careful not to tear the skin.
3. Mix the butter with the thyme, 2 tsp. salt, and a few grinds of pepper in a small bowl. Spread half of the butter inside the body cavity of the turkey. Put the lemon half, 3 sage sprigs, 3 rosemary sprigs, and about three-quarters of the remaining garlic cloves in the cavity. Put the remaining herb sprigs and garlic cloves in the neck cavity. Rub the remaining butter mixture over the skin.
4. Tie the legs together for a neater appearance, if you like. Tuck the wing tips behind the neck, securing any loose skin over the neck cavity beneath the wing tips.
5. Put the turkey, breast side up, in an If You Care Extra Large Parchment Roasting Bag, with seam side up on a large baking sheet. Don't push the turkey all the way to the back of the bag. Add the carrots, celery, shallots, and bay leaf. Seal the open end of the bag by folding tightly as many times as possible toward the top until it snugly fits the turkey.
6. Roast the turkey for 1-1/2 hours. Remove the turkey from the oven and raise the oven temperature to 375ºF. Cut or rip the top of the bag (do this carefully, as the steam will be very hot) to expose the turkey, and put it back into the oven. Roast uncovered until an instant-read thermometer registers 165°F in the thickest part of the thigh, about 30 minutes more.
7. With silicone oven mitts, carefully lift and remove the turkey from the bag. Transfer the turkey to a serving platter or cutting board, and tent it loosely with foil. Let the turkey rest for at least 30 minutes (and up to 1 hour) while making the gravy.
Make the gravy
1. Strain the juices from the bag through a fine-mesh sieve into a large saucepan, and discard the solids. Bring the juices to a simmer over medium-high heat.
2. In a small bowl, whisk the flour and the chicken broth together until the mixture is smooth. Slowly pour the flour mixture into the saucepan, whisking continuously until the gravy begins to thicken, 3 to 5 minutes. If the gravy gets too thick, thin it with more chicken broth until it reaches the consistency you like. Reduce the heat to medium, and continue cooking, whisking frequently, to eliminate any raw flour flavor, about 5 minutes more.
3. Whisk in the thyme, rosemary, sage, 1/4 tsp. black pepper, and the butter. Continue to whisk until the butter melts. Adjust the seasoning with salt, if necessary.
4. Carve the turkey and serve with the gravy.