This is an alternative way to fillet a flatfish, especially a large one -- something larger than 6 or 7 pounds. It is called the double fillet.
It is an important technique to learn if you find yourself catching or buying large flatfish with any regularity. Why? The larger the fish, the thicker and wider the backbone is. It can grow so thick that it will rob you of precious meat if you try to use the single-fillet technique we just went through.
First make a cut with the end of your fillet knife along the backbone. Where is it? Look for a thin, straight line on the fish that curves around the gut cavity. Cut there.