Remove and discard the leg truss from the turkey. Pull off and discard lumps of fat.
Remove giblets and neck. Rinse the bird inside and out; pat dry with towels.
Rub oil on skin. Squeeze juice from a lemon half into body cavity, then add the peel. Rub juice of remaining lemon half onto turkey skin.
Slide your fingers under turkey skin to gently loosen it (but leave in place) on breast, around outside of thighs and legs, and over back (from the neck end).
Push the rosemary-garlic mixture under skin, distributing evenly.
Place remaining garlic cloves and the rosemary sprigs inside the bird.
To roast, place turkey, breast up, on a V-shaped rack in a 12- by 17-inch roasting pan (or one that is at least 2 inches longer and wider than the bird). Roast in a 350 oven until a thermometer inserted through the thickest part of the breast to the bone reaches 160, 1 1/2 to 2 1/4 hours. Put on a platter and let stand up to 30 minutes; keep warm.
Skim and discard fat from pan juices. Reserve pan with juices for gravy.
