In a small bowl, mix together the Miso, rosemary, garlic, Dijon and red wine vinegar until smooth.
Spread the sauce over the leg of lamb, coating all sides and place in an airtight container. Allow to marinate for 4-8 hours.
Preheat a gas grill to it's highest setting until it reaches approximately 5-600°F. Sear the roast on all sides until golden (about 3-4 minutes per side). Lower the temperature to it's lowest setting (3-350°F) and allow roast to cook for another 25-30 minutes, turning once, until an internal temperature of 125-130°F is reached).
Remove from grill and allow to rest for 5 minutes before slicing. Slice and serve.