Preheat oven to 400 degrees F.

Rub off the excess papery skin from garlic without separating the cloves. Slice the tip off the head, exposing the ends of the cloves. Place the garlic on a piece of foil, drizzle with oil, and wrap into a package. Roast until the garlic is golden brown and soft, about 45 minutes. When cool enough to handle, squeeze the cloves and any oil left in the foil into a small bowl. Add chopped (or dried) herbs, salt, and pepper; mash with a fork to form a paste.

Scatter lemon slices and herb sprigs, if using, in a 9x13-inch roasting or baking pan. Loosen the skin on the turkey breast and spread all but 1 tsp. of the garlic paste under it, massaging the paste into the meat. Spread the remaining paste over the skin on the underside of the breast. Place the turkey in the pan and add 1 cup water.

Roast the turkey, checking the water every 30 minutes and adding more, 1/2 cup at a time, if the pan is dry, until an instant-read thermometer inserted in the thickest part (without touching bone) registers 165 degrees F, 1 1/4 to 1 3/4 hours.

Remove the turkey from the oven and let rest for 10 minutes. Discard the skin before carving. Skim and discard any fat from the pan drippings, then drizzle the drippings over the turkey, if desired (see Tip).