Heat oven to 450º F. Halve 1 lemon and rub it all over the lamb. Squeeze its juice into a small bowl; mix with the garlic and rosemary. Cut 10 slits, 1 to 2 inches deep, all over the meat. Stuff the cuts with the garlic mixture. Rub the meat with 1 tablespoon olive oil and season with 1 tablespoon salt.
Place the lamb in a roasting pan and roast for 30 minutes. Turn over and roast for 40 minutes more or until the thickest part of the meat registers 145º to 150º F. Remove from oven; let rest 10 minutes. Cut off 1/4 of the lamb meat and reserve for another use.
Meanwhile, in a large bowl whisk together the mustard, juice from the remaining lemon, remaining oil, and salt. Fold in the beans, shallot, and parsley. Reserve 2 cups of the beans for another use. Thinly slice the remaining lamb and serve with the white beans.