Preheat oven to 325 degrees F (165 degrees C). Spread butter generously over the lamb, and distribute garlic clove halves over the meat. Sprinkle with rosemary and pepper. Place the lamb on a rack inside a roasting pan. Roast the lamb in the preheated oven for 1 hour. Mix the lemonade concentrate and dry soup mix together in a bowl, pour over the lamb, and roast an additional 30 minutes. A meat thermometer inserted into the thickest part of the lamb should read 120 degrees F/58 degrees C for medium-rare or 145 degrees F/68 degrees C for medium-well. Allow the roast to stand for about 20 minutes before slicing.