To make the beans -- Tie the whole garlic cloves, bay leaves, peppercorns, and rosemary in a cheesecloth bag and put them in a large pot along with the beans and salt. Cover by 4 inches with water and bring to a boil over high heat. Reduce the heat to low, cover, and simmer until tender, 2 to 2-1/2 hours. (The beans should be very slightly soupy, so add a bit of water during cooking if they become too dry; if they seem too liquidy, drain off some of the liquid.).
Meanwhile, gently heat the 1/4 cup olive oil with the smashed garlic cloves until the garlic becomes fragrant. Remove from the heat and leave to infuse. When the beans are done, remove the cheesecloth seasoning bag. Before serving, take the smashed garlic from the olive oil and discard, stir the olive oil into the beans, and adjust the seasonings as needed. (The beans can be made the day before and reheated.).
To make the lamb -- Prepare a charcoal or wood fire in a grill or heat a gas grill to medium high. Brush the meat with the olive oil and, with a sharp knife, make slits about 1/2 inch deep all over the meat. Insert the garlic slices in the slits. Rub the meat with the salt and pepper and then sprinkle with the herbs, pressing them into the flesh with the palm of your hand.
When the coals have become coated with white ash, oil the rack and grill the lamb for about 10 min., paying attention to flare-ups so it doesn't char or overcook. Turn and cook another 10 to 20 minute or until an instant-read thermometer inserted in the thickest part of the meat registers 130°F for medium rare. Transfer the lamb to a carving board or platter (one that will collect the juices) and loosely cover it with foil. Let stand for 10 minute before carving into thin slices. Serve with a little of the carving juices spooned over, accompanied by the white beans, drizzled with more olive oil, if you like.