Mix together the butter, chopped rosemary, lemon zest and juice until you have a thick paste and set aside.
Season steaks with salt and pepper on both sides. Pan-fry one at a time, or if you have a large skillet, pan-fry both at the same time.
Heat the olive oil in a large pan over high heat. Once very hot, add steaks to the pan and cook for 2-3 min on each side for medium-rare. Cook for a further 1 minutes for well done. When you turn the steaks for the first time, add in the butter paste and garlic cloves on the top of the steaks, basting the steaks from time to time with the melted butter paste. Just before you remove the steaks from the pan, add in the balsamic vinegar and allow it to cook for a minute or so.