Slice off the head using a sharp knife. With a sharp pair of scissors, cut off the tail and fins. Make an incision down the centre of the fish from the head to the tail.
Starting at the incision in the centre of the fish, use the tip of the knife to gradually cut the fish from the bone with long sweeping movements, working until the knife reaches the outer edge of the fish.
Repeat the process three more times, removing the fillet opposite the first one, and the two fillets on the other side of the fish.
To remove the skin from a fillet, place it skin-side down onto a work surface and secure the tail-end with your fingertip.
Make a small incision between the flesh and the skin at the tail-end. With the knife moving away from you and the blade slightly angled towards the skin, cut the flesh from the skin in a sawing motion.
To make the sauce
In a small bowl, stir together butter, parsley, lemon zest and juice, horseradish and mustard.
Season both sides of the fillets with salt. Lay flat on a clean work surface. Reserving 2 teaspoons of the butter mixture, divide evenly on top of the fillets. Fold fillets in half, enclosing the butter mixture. Place the fillets in an oven-proof dish.
Sprinkle the breadcrumbs over each fillet and bake in a preheated oven at 180°C for 10-12 minutes or until the fillets are cooked, the butter has completely melted and the breadcrumbs have toasted.
Serve with steamed green vegetables and roasted potatoes.
Notes
Take your time when preparing the fish. This is a tricky skill, but impressive once you have mastered it.