Place a large non-stick frying pan on a medium-high heat. Rub the steak all over with a pinch of sea salt and black pepper, then sear on all sides for 10 minutes in total, so you achieve good colour on the outside but keep it medium rare in the middle, or cook to your liking, turning regularly with tongs.
Meanwhile, strip the rosemary leaves off the sprigs, peel and finely slice the garlic, and tear up any larger mushrooms. When the steak’s done, remove to a plate and cover with tin foil. Reduce the heat under the pan to medium, and crisp up the rosemary for 30 seconds, then add the garlic and mushrooms and cook for 8 minutes, or until golden, tossing regularly. Pour in the beans and their juice, add 1 tablespoon of red wine vinegar and simmer for 5 minutes, then season to perfection. Sit the steak on top and pour over any resting juices. Slice and serve at the table, finishing with a little extra virgin olive oil, if you like.