Heat a large pan on a medium-high heat.
Rub the steak with a pinch of salt and black pepper, then sear the steak for 4 minutes on each side, remove the steak from the pan, cover with tin foil and rest for 5-10 minutes.
Reduce the heat to medium, and in the same pan add the mushrooms and cook for 5 minutes until starting to brown. Add the rosemary leaves and the garlic slices to the mushrooms and cook for a further minute, stirring constantly to stop the garlic from burning.
Now add in the cannellini beans and their juice, as well as the vinegar. Season to taste with salt and pepper and simmer for 5-6 minutes.
Once ready, pour the steak juices on top of the bean and mushroom mixture and give it a quick stir to combine.
Now divide the bean and mushroom mixture between 2 bowls. Take the steak and cut it into slices. Lay the sliced steak over the beans, splitting the portion of meat between bowls. Serve immediately.