Preheat oven to 425°F with rack in lowest position.
1. Stir together butter, sage, lemon zest, and salt in a small bowl.
2. Starting at wider, thicker end of breast, gently slide an index finger between skin and flesh of breast to loosen skin, leaving skin attached to breast at other end (be careful not to tear skin). Push butter mixture evenly under skin on both sides of breast, and massage skin from outside to distribute butter evenly.
3. Put turkey on V rack in roasting pan and sprinkle all over with pepper. Add 2 cups water to pan and roast turkey 30 minutes. Reduce oven temperature to 350°F. Rotate breast 180 degrees and baste with pan juices, then roast 30 minutes. Rotate breast again 180 degrees, and baste with pan juices. Tent breast loosely with foil and continue to roast until an instant-read thermometer inserted into thickest part of breast registers 165°F. Remove from oven and loosely cover with foil. Let rest 10-15 minutes. Serve.