Make sure that the knife is really sharp. First remove the fillet on the left side of the fish. To do this, hold the knife very flat, put the tip of the first finger - or thumb, if it feels more convenient - into the cut down the backbone, draw back the flesh with the left hand, cutting it off the bones with the knife (see Fig. I). Make long, smooth cuts with the knife, always cut-tingtowards you. Be careful that with each cut the knife feels the bone, then no flesh will be left on it. Continue to draw back the flesh, tad cut it away from Unbone until one fillet is cut right off. Lay it on a tin lined with kitchen paper.
Now turn the fish so that the second fillet is on your left hand, then remove it in the same way as the first. When this is done, the upper part of the fish will have all the bones exposed (see Fig. 2, a). Next, turn the fish over and remove the two under fillets in exactly the same way, remembering the fillet that is being removed must always be on your left hand.
There will be four fillets in all, and if the operation has been neatly performed, the skeleton will be complete, with no gaps in the fin bones round.
If, however, there are some bones missing from the skeleton, feel carefully over the fillets. They will probably be still adhering to the flesh, so cut them off carefully. Put all the bones and fish trimmings in a saucepan with water to well cover them, and let them cook steadily for a quarter of an hour or a little more; then use the stock as the foundation of the sauce to serve with the fish.
Cut each fillet in two or three pieces, according to its size. It is best to cut in a slanting direction; the pieces are then a better shape than if the fillets were cut straight across.