In a blender or food processor, pulse the first 8 ingredients until finely chopped. In a 15x10x1-in. baking pan, open roast flat. Spread herb mixture evenly over meat to within 1/2 in. of edges; sprinkle lemon zest over herb mixture., Starting at a long side, roll up jelly-roll style. Using a sharp knife, score fat on outside of roast. Tie at 2-in. intervals with kitchen string. Secure ends with toothpicks. Refrigerate, covered, at least 4 hours or overnight., In a 6-qt. slow cooker, combine onion and wine. Place porchetta seam side down on top of onion. Cook, covered, on low 5-6 hours or until tender. Remove toothpicks. Reserve cooking juices., In a large skillet, heat oil over medium heat. Brown porchetta on all sides; remove from heat. Tent with foil. Let stand 15 minutes. , Meanwhile, strain and skim fat from cooking juices. Transfer to a large saucepan; bring to a boil. In a small bowl, mix cornstarch and water until smooth; stir into juices. Return to a boil, stirring constantly; cook and stir until thickened, 1-2 minutes. Cut string on roast and slice. Serve with gravy.