Filleting a Whole Flatfish
1. Using kitchen shears, remove fins and the tail. Use the shears to cut out the gills under the flap at the base of the fish’s head. Discard fins and gills.
2. With the fish’s head facing away from you, cut down to the backbone along the line that separates the two fillets (the lateral line).
3. Press the knife against the backbone so that the blade rests flat. Slide the knife under the fillet to your left in long swiping motions, moving from head to tail. Repeat until you’ve detached the fillet. (Right-handed people should cut to their left; left-handed people should cut to the right.)
Thanks to James Peterson for these instructions from his book “Fish & Shellfish”.