Combine the first 6 ingredients in a large shallow non-reactive container. Place lamb in marinade and let marinate in the refrigerator for 2 to 3 hours.
Cook the garlic in olive oil in a skillet on the stove for 15 to 20 minutes, or until soft and fragrant. Remove from heat and let cool slightly. Add garlic-olive oil mixture and all remaining ingredients to a food processor and process until thoroughly combined. Keep refrigerated until ready to use.
Preheat a grill over medium heat. Remove lamb from marinade and grill lamb racks until desired degree of doneness. Top with the Roasted Garlic Rosemary Butter.