Chop garlic. Pat lamb dry and season with salt and pepper. In a 12-inch heavy skillet heat oil over moderately high heat until hot but not smoking and sear lamb 3 minutes. Turn lamb over and sear 2 minutes more for medium-rare. Transfer lamb to 4 plates. Add garlic, rosemary, lemon juice, and stock to skillet and deglaze over high heat, stirring and scraping up brown bits, 1 minute. Whisk in butter. Pour sauce over lamb.