2. Place the lamb in a resealable bag and add the marinade. Seal the bag and marinate the lamb overnight in the refrigerator.
3. Preheat the oven to 375ºF.
4. Cut the potatoes in half (or in quarters if large). Toss the potatoes with the olive oil, salt and pepper and place them on a baking sheet.
5. Roast the potatoes in the oven until they are tender, about 45 minutes.
6. Meanwhile, preheat the grill to high.
7. Remove the lamb from the bag (discard the leftover marinade), season the meat with salt and pepper and turn the grill down to medium. Grill the lamb until a meat thermometer registers 140ºF, about 10 to 15 minutes per side, depending on the thickness of the lamb.
8. Transfer the meat to a carving board, let it rest for 10 minutes and slice it on the bias.
9. Transfer the potatoes to a serving platter and sprinkle with Parmesan.
10. Serve the sliced lamb with the potatoes on the side.