Preheat oven to 425 degrees F (220 degrees C).

Place leg of lamb in a large baking dish. Prick in 6 places with a sharp knife; insert garlic cloves into the slits. Brush butter over lamb. Season with salt and pepper.

Roast in the preheated oven, basting regularly with juices, for 25 minutes. Arrange cannellini beans around the lamb, mixing to coat with juices. Continue roasting until beans are warmed through and the internal temperature of the lamb reaches 160 degrees F (71 degrees C) for medium, about 15 minutes more.

Let lamb rest for 10 minutes before slicing. Garnish with chopped parsley and serve beans on the side.