In a large bowl, whisk together the maple syrup, oil, soy sauce, vinegar, mustard and garlic. Add the lamb, turn to coat, then cover and chill for 1 hour or up to 2 days, turning occasionally.
Take the lamb out of the refrigerator 30 minutes before grilling. Preheat a grill to medium-high. Season the lamb with the salt and pepper and grill. Cover the lamb with foil and cook for 12 minutes, rotating once. Turn the lamb over, cover and cook for 12 to 15 minutes more, until the internal temperature reads 130 degrees on a meat thermometer inserted into the thickest part of the lamb. Transfer the lamb to a cutting board, cover loosely with foil and let stand for 5 to 10 minutes before slicing.