Put the butter beans, 4 tbsp of the oil, the garlic and 3 tbsp of water into a pan and heat through. Smash the beans with a potato masher until you have a rough mash, stir in the lemon juice and mint, season to taste and keep warm.
Season each lamb steak with the black pepper and a sprinkling of salt. Heat the remaining oil in a large frying pan until really hot. Add the lamb steaks and cook for 3 minutes on each side for pink meat or until cooked to your liking. Spoon the mash onto four warm serving plates and top each with a lamb steak. Quickly stir half a cup of water into the lamb pan, boil to make a sauce and spoon over the lamb. Add a handful of watercress to each plate and serve.