In a medium bowl, combine butter, miso, ginger, garlic, rice vinegar and honey. Stir together to make a paste
Massage paste onto rack of lamb using your hands and place skin side up in an oven proof dish. Sprinkle with pepper and roast for 15 minutes
After 15 minutes, cover loosely with foil and continue to cook for an additional 15-20 minutes or until the lamb reaches an internal temperature of 140ºF (60ºC) for medium doneness
Remove from oven and allow it to sit for 10 minutes before slicing
Top with scallions and sesame seeds (optional) and serve