1. For the turkey: Let the turkey sit out for about 1 hour to come to room temperature. Preheat the oven to 400°F. Remove the packet of neck and giblets and set aside. Cut away any fatty pieces near the cavity or neck of the bird and discard. Pat the turkey dry inside and out with paper towels and sprinkle liberally with salt and pepper inside and out. Combine the butter and chopped rosemary. Loosen the skin over the breast of the turkey and push half of the butter mixture under the skin and over the breast meat. Rub the skin of the turkey with the rest of the butter mixture. Stuff the cavity of the turkey with the lemons and rosemary sprigs. Tie the legs together with twine. Tuck the wings under the body of the turkey and place it, breast side up, on a rack in a roasting pan.
2. Roast the turkey until it begins to turn golden, about 25 minutes. Loosely tent the breast with foil, then reduce the oven temperature to 325°F. Continue cooking, basting the turkey with pan drippings every 30 minutes, until the internal temperature near the thigh joint reaches about 155°F. Remove the foil and continue roasting until it reaches 165°F., about 30 minutes longer. Transfer the turkey and rack to a cutting board and let rest about 30 minutes while you make the gravy.
3. For the gravy: Using a fat separator or spoon, remove all but 2 tablespoons of the fat from the drippings in the roasting pan. Transfer the drippings and any brown bits from the bottom of the pan to a large straight-sided skillet. Set over medium-high heat. Add the giblets and neck and cook until nicely browned, about 5 minutes. Add the carrot, celery, onion, sage and rosemary. Lower the heat to medium-low and continue cooking until the vegetables are softened and lightly browned, about 10 minutes. Stir in the marsala, scraping up any browned bits from the bottom of the skillet. Simmer until the marsala is reduced by half. Slowly sprinkle in ¼ cup of the flour, stirring constantly to avoid lumps. Cook, stirring, 4 minutes. Pour in the chicken broth and simmer, whisking occasionally, until the gravy is thickened, about 15 minutes. Taste the gravy and add salt, pepper and lemon juice to taste. Strain the mixture through a fine-mesh sieve into a medium saucepan; discard the solids. Bring the gravy to a boil over medium heat. Mix the remaining 2 tablespoons of flour with the butter in a small bowl until it forms a paste. Drop pea-size pieces of the mixture into the gravy, whisking vigorously to incorporate between additions, until all the butter mixture is incorporated. Continue cooking until the gravy thickens enough to coat the back of a wooden spoon, about 5 minutes more.
4. To serve: Present the turkey whole on a platter garnished with the lemons from inside the cavity. To carve, first remove the wishbone, then legs, thighs and wings, and thinly slice the breast meat on an angle. Serve with the gravy on the side.
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