Clean the heart under cold running water until the water runs clear.
Cut the heart in half lengthwise so that you have two squares. Trim off the outer white membrane.
Cut the squares into strips and the strips into small squares.
Marinate the squares in the balsamic, oil, rosemary, and salt and pepper, covered in plastic for at least an hour.
Heat a pan over high heat with a bit of oil and quick sear the heart squares, about 1 minute on each side. You want them to be no more than medium rare. Serve immediately!