In a medium pan over medium heat, cook the onion in half of the olive oil for 10 minutes, until it's soft and slightly brown.

Add the vinegar and cook another five minutes.

Heat a pot with the remaining olive oil over medium heat. Add the garlic, beans and rosemary and cook for five minutes. Use a potato masher or fork to roughly mash the beans. Add lemon juice and salt and pepper to taste.

Heat a frying pan over high heat. Season the steak with salt and pepper and cook for two to three minutes per side until medium-rare. Slice and serve over the beans and spoon the onions on top.