Cut potato in 4, lengthwise. Place each piece flat on a cutting board. With a paring knife, make a very thin cut lengthwise down the center of each piece, being sure to avoid cutting the edge. Make another thin cut right beside the first cut then gently use your knife to remove the piece of potato, creating a groove in the middle of the potato that becomes the “backbone” of the filet. Make shallow, closely spaced diagonal lines on each side of the backbone, that will become the “bones”. Set aside in a shallow bowl.