Grate the zest from one lemon and combine with the thyme, salt, garlic, and pepper in a small bowl; stir in the olive oil until blended. Rub the entire surface of the pork to coat with the herb mixture.
Place roast top side up in a small roasting pan. Roast 30 minutes, then turn pork over, baste with pan juices and roast 30 minutes. Turn top side up; baste; roast 15 minutes or until center of roast reads 140°F
Remove from oven, cover loosely with foil and let stand 15 minutes. Center temperature should have risen to 150°F
Carefully remove strings and cut into thin slices.