If fillets are needed, the fishmonger will always fillet the fish chosen; but it is more economical to do this at home, as the bones and trimmings may be used for fish stock and for soups and sauces.

To fillet a sole, lemon sole, or plaice, skin the fish and lay it flat on the table, and with a cook’s knife cut the flesh right down the backbone and then round the fins. Ease the flesh from the backbone by means of the knife, and then, so to speak, peel the four pieces of fish away from it. A small sole will make four fillets. If a large fish is used, these four fillets may be divided into eight. A cook’s knife should be used, and it must be extremely sharp, otherwise it will not be possible to skin and fillet neatly.

In filleting whiting, haddock and fish of that description, remove the heads and fins, split the fish open, remove the backbone and divide the fish into two fillets, and these into smaller pieces if required.

— Fish and How to Cook It by Mrs. C. S. Peel (1907)