Lay the fish on a board with the back away from you and the head pointing to the left. Lift the gill fin and cut at an angle behind the fin to the top of the head. Hold the fish firmly against the board. Insert the knife at the head end and keeping the knife almost flat cut along the top of the back of the fish to the tail.
About halfway down the fish, near the end of the belly cavity, push the knife blade through and over the backbone. Cut towards the tail keeping the knife as flat to the bone as possible. Lift this part of the fillet up and, using long sweeping strokes and keeping the knife blade almost flat, cut the top half of the fillet free from the rib cage.
Turn the fish over with the head pointing to the right. Insert the knife at the tail end and make a long cut along the top back of the fish towards the head. Lift the gill fin and make an angled cut around the head. Repeat the process as before finishing by cutting the fillet free from the rib cage.